Remove support for PostgreSQL 11 and 12 (#18034)

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. 😄

.ci/scripts/calculate_jobs.py

@@ -60,7 +60,7 @@ trial_postgres_tests = [
     {
         "python-version": "3.9",
         "database": "postgres",
-        "postgres-version": "11",
+        "postgres-version": "13",
         "extras": "all",
     }
 ]