My understanding is that postgres doesn't need to be upgraded. It'll still work with version 15 or whatever you have. Postgres 15 has some sort of memory leak that they're trying to get away from, so they made 16 the new default.
Same with the "requirement" to upgrade pict-rs to the latest version. You can keep the old version if you don't care about the new image proxy feature.
Really it's a not a problem of needing to upgrade this stuff, but a problem with the documentation which isn't clear. That's a big weak spot for the Lemmy project in my opinion. I only learned the above information from lurking a bit in the Matrix chat.
Ah, interesting. That experience combined with the wording of "this update requires postgres 16", I can see where the confusion comes from.
Interestingly enough, they fixed the documentation. Now it says "We recommend upgrading to PostgreSQL 16 due to a known memory leak in PSQL 15. To use the new image proxy feature, pict-rs version 0.5+ is required." https://join-lemmy.org/news/2024-06-07_-_Lemmy_Release_v0.19.4_-_Image_Proxying_and_Federation_improvements