Newer Postgres major versions

Goal

Support newer Postgres versions: 13, 14, 15(beta)

TODO / How to implement

• Currently, repot A002 says "Consider upgrading to the newest major version: 12" – there should be 14 as the latest available major version (not in beta). See https://gitlab.com/postgres-ai/postgres-checkup/-/blob/master/pghrep/internal/checkup/a002/a002.go
• Adjust CI to test for newer PG versions: 13, 14 (15beta2 too but only if it's a low-hanging fruit; if it's not, create another issue for PG15)
• test properly and ensure that postgres-checkup works well with 13 and 14

Acceptance criteria

• I can run postgres-checkup for Postgres 13, 14, and (maybe) 15beta2
• If I run postgres-checkup for older Postgres versions it suggests upgrading to newest PG major version, 14.