Your question is twofold.
is there any official mention of which version the Blade Runner 2049 uses as its definitive source?
The best "official mention" is in this Collider interview, where director Denis Villeneuve has this to say (text version of the relevant part quoted from this ScreenRant article):
I think honestly I would recommend seeing the very, very final director’s cut of Ridley’s. If they are curious enough, for me, it’s like the very first cut that was released in 1982 with the voice over and the very last one. The one with the voice over is the one that I was raised with, that made me discover this universe. At the time, I was not aware of the controversy… I saw the movie in a small town in Canada, there was no internet at the time, so I didn’t know that critics didn’t welcome the movie well. I just deeply loved the movie at first sight, and for me, the voice over had that kind of noir quality, so it’s not something that shocked me, I embraced it at the time.
Now, I understood, much later, when I saw Ridley’s final director’s cut, I understood where he wanted to go and what he was missing. Then, I became a fan of it, so my favorite version is the final director’s cut from Ridley.
This "final director's cut" is the 2007 Final Cut. So there you have it: The director of Blade Runner 2049 thinks you should see the Final Cut of Blade Runner – because it's his favorite.
That doesn't mean he used the Final Cut as his definitive source for the sequel, however. This answer at the Movies & TV Stack Exchange, mentioned in comments above, has some more interesting quotes from Villeneuve. I'll quote it here in full:
So when Blade Runner 2049 director Denis Vileneuve met with reporters at San Diego Comic-Con, someone inevitably had to ask which version of Blade Runner is Blade Runner 2049 a sequel to. "The thing is that I was raised with the first one," Vileneuve said, referring to the original theatrical cut. "There was one Blade Runner at the time. I remember seeing the first movie and falling deeply in love with it."
But his love for the original doesn't mean that there isn't room for nuance, or that he can't acknowledge the intent of Scott's later versions of the film. "The key to making [Blade Runner 2049] was to be in between," Vileneuve said. "[The theatrical version] is the story of a human falling in love with an artificial being, and the story of [the director's cut] is a replicant who doesn’t know he’s a replicant and slowly discovers his own identity. Those are two different stories."
In order to tell his story without alienating fans who prefer one version of the story, the director went back to the source material, Philip K. Dick's Do Androids Dream of Electric Sheep? for inspiration. "I felt that the key to deal with that was in the original novel," Villeneuve said. "In the novel the characters are doubting themselves and they aren’t sure if they are replicants or not. From time to time the detectives are running scans on themselves to make sure that they are human. I love that idea so I decided that in the movie Deckard is unsure, as we are, of what his identity is. I love mystery."
As for which version Harrison Ford and Scott consider to be the truth? "Harrison and Ridley are still arguing about that," Vileneuve says. "If you put them in the same room, they start to talk very loudly about it."
So that's vague, but the take-away from that is that the Theatrical Cut and the Final Cut (the only two major cuts of the movie) tell two different stories, and Villeneuve didn't want to consider one of them more canon than the other.
Now to your subjective question:
what version of the original Blade Runner should I have seen in order to be up-to-date with Blade Runner 2049?
In my opinion: It doesn't matter.
It is important to have seen a version of Blade Runner to understand several references and plot points in Blade Runner 2049. None of those plot points are unique to any of the versions of the original movie, however. The plot points that are unique to the Final Cut aren't that important in the sequel, or are disregarded.
The ending of Blade Runner's Final Cut is more ambiguous than the ending of the Theatrical Cut. I can't really discuss the details of that without spoiling either movie, but I will say that I think the Final Cut is a much more interesting movie than the Theatrical Cut, even if the ambiguity might be somewhat resolved by the sequel. Spoilers for the ending of Blade Runner's two main versions:
In the Final Cut of Blade Runner, it's implied that the blade runner Rick Deckard himself might be a replicant. In the Theatrical Cut, there's nothing to imply that he's anything but a human.
The sequel seems to resolve this ambiguity. Spoilers for 2049:
We meet an older Deckard who has aged like a human. There's no mention of him being a replicant.
Superficially, this seems to comply with the ending of the Theatrical Cut, and to disregard the ambiguity raised by the Final Cut. However, the ambiguity still exists, if you stretch your headcanon a little. And if I understood Villeneuve correctly in the Collider interview I quoted at the top, that's exactly what he intended for us to do to make it fit with either version. Spoilers for several parts of 2049:
Deckard has lived in an irradiated area for years, seemingly without having taken any damage. It's also not clear to me whether Tyrell's experimental replicant breeding program was meant to allow humans to breed with replicants, or simply to allow replicants breeding with replicants (by biological definition, are the more advanced replicants the same species as humans, and can interbreed with them, or not?). In his meeting with Deckard, Wallace seems to imply that Tyrell might have set up Rachael and Deckard's romance in order to make them procreate, a scenario which works either way, but perhaps best if Deckard is a replicant that Tyrell could, in some capacity, control.
All that said, I agree with Villeneuve in that I would recommend watching the Final Cut of the original movie.