Run integration tests against postgres 14 (go-gitea#34514)

postgres 12 is end of life since 6 months. 13 and above are still
supported but I think it's overall better if we test a more recent
version of postgres because that's what new users will be running on.

Ref: https://endoflife.date/postgresql

Author: silverwind

.github/workflows/pull-db-tests.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     services:
       pgsql:
-        image: postgres:12
+        image: postgres:14
         env:
           POSTGRES_DB: test
           POSTGRES_PASSWORD: postgres