Run PostgreSQL 11 jobs when pipeline:run-on-pg11 label is applied

What does this MR do and why?

This MR runs PostgreSQL 11 jobs in Merge Requests pipelines when ~"pipeline:run-on-pg11" label is applied to the merge requests.

Closes #385781 (closed)

Testing

An example pipeline: https://gitlab.com/gitlab-org/gitlab/-/pipelines/727434514.

I added the label ~"pipeline:run-on-pg11" to !107432 (closed) and verified that rspec db-library-code pg11, rspec migration pg11, rspec unit pg11, rspec integration pg11, and rspec system pg11 are created at https://gitlab.com/gitlab-org/gitlab/-/pipelines/727434514.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.