On next release, include upgrade instructions for postgres 15 -> 16

[dessalines]

This is unfortunately necessary due to a postgres 15 memory leak: LemmyNet/lemmy#4406

This PR has the file: https://github.com/LemmyNet/lemmy/pull/4426/files#diff-4bb683dcf59cc3a55cf3b7c6f0b6a202149806f28f6ab704a83050d7d3aabf6c

Once that's merged, we need to make sure to have instructions for the upgrade on the next lemmy-ansible release.

[ticoombs]

So this also removes the need to have jit = no in our customPostgres file too.

[dessalines]

I don't know enough about that setting to say if we should keep / remove, but I'd say pry remove.

[ticoombs]

lemmy.cafe dev/admins "confirmed" that the leak no longer is present when using jit=no so we can get rid of that too.