Enable pg13 auto-upgrade for single node installs in 15.11

Postgres 13 is going to become our minumum supported version in GitLab 16. We should turn on the auto-upgrade for single node installs in 15.11

• Swap attempt_auto_pg_upgrade? to true in https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/files/gitlab-ctl-commands/upgrade.rb
• Swap ATTEMPT_AUTO_PG_UPGRADE to true in https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/docker/assets/wrapper
• Ensure the pg-upgrade upgrade command is working as expected
• Update the documentation to reflect these changes.

The last time we did this was !4848 (merged)