postgresql_12: remove

This will be EOL at the end of November, so there's little reason to
keep it in 24.11[1]. As discussed, we'd like to keep it for as long as
possible to make sure there's a state in nixpkgs that has the latest
minor of postgresql_12 available with the most recent CVEs fixed for
people who cannot upgrade[2].

This aspect has been made explicit in the manual now for the next .11
release.

During the discussions it has been brought up that if people just do
`services.postgresql.enable = true;` and let the code decide the
postgresql version based on `system.stateVersion`, there's a chance that
such EOL dates will be missed. To make this harder, a warning will now
be raised when using the stateVersion-condition and the oldest still
available major is selected.

Additionally regrouped the postgresql things in the release notes to
make sure these are all shown consecutively. Otherwise it's a little
hard to keep track of all the changes made to postgresql in 24.11.

[1] https://endoflife.date/postgresql
[2] https://github.com/NixOS/nixpkgs/pull/353158#issuecomment-2453056692

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