Remove support for PostgreSQL 11 and 12

[clokep]

This is essentially matrix-org/synapse#14392. I didn't see anything in there about updating sytest or complement.

The main driver of this is so that I can use jsonb_path_exists in #17488. 😄

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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  (run the linters)

[clokep]

I think the failures are flakes, I had a full green run on my account: https://github.com/clokep/synapse/actions/runs/12358363779

[anoadragon453]

This LGTM. Thanks for adding to the upgrade notes as well!

I didn't see anything in there about updating sytest or complement.

Complement: We have a Synapse Complement container image, which is on PG13 already.

Sytest: Sytest's CI makes use of the sytest-synapse container image, which is built using Debian bullseye. Bullseye uses postgres13.

So we're good for this upgrade, but will need to edit CI config in the case of Postgres 14+!

[clokep]

Thank you!