Rollout multi-DB CI testing for gitlab_sec

Problem to solve

Before Phase 4 of the production rollout of gitlab_sec we need to switch our default CI configuration over to using multiple DB connections by default

• Testing for multiple databasees docs https://docs.gitlab.com/ee/development/database/multiple_databases.html#testing-for-multiple-databases
• Related CI work Fix spec failures on CI when two connections (#391646 - closed)
  • Script for aggregating failures Fix spec failures on CI when two connections (#391646 - closed)
  • Pipeline enablement Test jobs with single db, two connections (!112383 - merged)