ci: Update default Postgresql to v14

v14 is the new default for Omnibus installations: https://docs.gitlab.com/ee/administration/package_information/postgresql_versions.html

We still maintain support for Postgres 11 for users who aren't using the same version of Postgres shipped with Omnibus: https://docs.gitlab.com/ee/administration/gitaly/praefect.html#preparation

Update the CI config to reflect this.