Deprecate custom SSH client configuration for the git user

What does this MR do?

Documents that adding custom SSH configuration for the GitLab-managed 'git' user is strongly discouraged, and adds a system check that will warn if custom configuration is present

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

We need to ensure this is merged in time for %10.0 and noted in the release post, so we can remove support in 10.x

Why was this MR needed?

Custom SSH client configuration may be available to all users of the GitLab instance, which is surprising in some cases. If private keys are added without full knowledge of this, it may be a security risk.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37204 (closed)