Warn for PostgreSQL 13 deprecation (#19170)

[3nprob]

This reverts commit 5d545d1 and introduces a deprecation warning for PostgreSQL 13.

Would give users some heads-up and unblock upgrading synapse for now.

#19170 (comment)

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[anoadragon453]

@3nprob our dependency deprecation policy states that we'll drop support for Postgres and Python versions once they go end-of-life. The end-of-life date for PostgreSQL 13 has been known for over 5 years.

Perhaps it would have been helpful if we announced that PostgreSQL 13 would be dropped in the next release in 1.142.0's release notes? I'm happy to make that a policy going forwards.

I don't believe we're actually taking advantage of any features that require PostgreSQL 14+ yet, so the block is currently arbitrary and could be removed. But anyone that did want to add features requiring PG14+ would need to be aware that we're waiting an extra release or two, and hold off on their changes.

I would be more sympathetic if 1.143.0 contained a security fix and updating to it was mandated (though generally in that case we'd provide a 1.142.1 security patch).

For now I suggest you pin your Synapse version or patch 1.143.0 to remove the block locally until you're able to upgrade your PostgreSQL cluster to a supported version.

[anoadragon453]

Going to close this to take it off the review queue. But please feel free to continue to leave a comment if you feel I'm being unreasonable.

Perhaps it would have been helpful if we announced that PostgreSQL 13 would be dropped in the next release in 1.142.0's release notes? I'm happy to make that a policy going forwards.

We've committed to doing this in #19239,