Resolve "Removed omniauth provider causing invalid application setting"

What does this MR do?

We now remove unrecognized omniauth providers instead of raising a validation error.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Because if the list of disabled providers contains providers that are no longer present in config/gitlab.yml, you are not able to update GitLab settings.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46783 (closed)