PG-1900 Remove compatibility with PG-12

PG-12 reached EOL one year ago, so we can cleanup compatibiliby code for
that version and remove related CI workflows. This change requires
migration script to remove pgsm_create_11_view SQL function and as a result it
requires extension version bump.

regression/expected/version.out

@@ -2,7 +2,7 @@ CREATE EXTENSION pg_stat_monitor;
 SELECT pg_stat_monitor_version();
  pg_stat_monitor_version 
 -------------------------
- 2.2.0
+ 2.3.0
 (1 row)
 
 DROP EXTENSION pg_stat_monitor;