Test logging in via the Authentiq OAuth provider in a feature spec

What does this MR do?

• The migration:path-pg build was previously failing when the Authentiq feature spec was enabled by placing Authentiq configuration in the test section of gitlab.yml

• The migration:path-pg task checks out an old revision of the codebase (v8.14.10) and runs a schema:load. It then checks out the commit under test, and runs db:migrate, to verify that migrations run without errors.

• The problem here is that v8.14.10 does not have the Authentiq module installed, but is run with the gitlab.yml for master, which would contain the Authentiq configuration in the test section.

• The solution was to use the v8.14.10 gitlab.yml for the schema:load, rather than the gitlab.yml from master.

What are the relevant issue numbers?

• Closes #34737 (closed)
• EE conflicts fixed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2368

Tasks

• Investigation
• Implementation
• Tests
  • Added
  • Passing
• Meta
  • CHANGELOG entry created
  • Branch has no merge conflicts with master
  • Squashed related commits together
  • Check for clean merge with EE
• Review
  • Reviewer
  • Maintainer
• Wait for merge

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/cc @alexkeramidas