Remove support for PostgreSQL 13 (#19170)

This PR removes support for PostgreSQL 13 as it is deprecated
(tomorrow).
Uses https://github.com/element-hq/synapse/pull/18034 as a reference of
where to look, and also found a few other places that needed updating.
I didn't see anywhere in Complement that needs updating.
There is a companion Sytest PR deprecating psql13 over there:
https://github.com/matrix-org/sytest/pull/1418

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.ci/scripts/calculate_jobs.py

@@ -72,7 +72,7 @@ trial_postgres_tests = [
     {
         "python-version": "3.10",
         "database": "postgres",
-        "postgres-version": "13",
+        "postgres-version": "14",
         "extras": "all",
     },
     {