ci: Test low-level database library code against PG12 [RUN ALL RSPEC]

What does this MR do?

• Add {,ee/}spec/support/helpers/database/**/* to .db-patterns
• Add a new .db-library-patterns pattern config for the following file regexs: "{,ee/}{,spec/}lib/{,ee/}gitlab/database/**/*", "{,ee/}{,spec/}lib/{,ee/}gitlab/database{,_spec}.rb", "{,ee/}spec/support/helpers/database/**/*"
• Add a new rspec db-library-code pg12 job that runs tests for low-level DB code against PG12 (note that these are already tested against PG11 in the rspec unit pg11 / rspec-ee unit pg11 jobs).

Example job: https://gitlab.com/gitlab-org/gitlab/-/jobs/945271646

Post-merge tasks

• Post to #development
• Post to the Engineering Week in Review

Related to gitlab-org/database-team/team-tasks#110 (closed).