Resolve "undefined method `new_project_guidelines_html', unable to load from cache"

What does this MR do?

    # Clears the current Appearance cache otherwise it breaks since
    # new_project_guidelines_html would be missing. See
    # https://gitlab.com/gitlab-org/gitlab-ce/issues/41041
    # We're not using Appearance#flush_redis_cache on purpose here.

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

We're not using Appearance#flush_redis_cache on purpose here because if the method is removed in the future, the migration would break.

Using Rails.cache.delete is safe since it will do nothing if the cache is empty.

Does this MR meet the acceptance criteria?

• No need for a Changelog entry since it solves a fix introduced in a RC
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #41041 (closed)