Add cop to catch when using stub_feature_flags in before_all

Description of the proposal

Add a new cop to catch the usage of stub_feature_flags in before_all.

Using stub_feature_flags in before_all leads to incorrect behaviour when there're more than one linked examples, since stub_feature_flags is called once before the first example, but the feature flag state will be reset after every example https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/spec_helper.rb#L470.

The new cop discourages using stub_feature_flags in before_all hook and instead recommends using it in before hook.

Check-list

• Make sure this MR enables a static analysis check rule for new usage but ignores current offenses.
• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend).
• [-] If there is a choice to make between two potential styles, set up an emoji vote in the MR:
  • CHOICE_A: 🅰️
  • CHOICE_B: 🅱️
  • Vote for both choices, so they are visible to others.
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus).
• [-] (If applicable) One style is getting a majority of vote (compared to the other choice).
• [-] (If applicable) Update the MR with the chosen style.
• Create a follow-up issue to fix the current offenses as a separate iteration: ISSUE_LINK
• Follow the review process as usual.
• Once approved and merged by a maintainer, mention it again:
  • In the relevant Slack channels (e.g. #development, #backend, #frontend).
  • (Optional depending on the impact of the change) In the Engineering Week in Review.

/cc @gitlab-org/maintainers/rails-backend