Resolve "Add a 'custom cop' to RuboCop that bans ENV assignment"

What does this MR do?

This adds a new RSpec::EnvAssignment that forbids calling ENV#[]= in specs and autocorrects to use stub_env instead.

Does this MR meet the acceptance criteria?

• 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
• If the ee_compat_check failed, an EE compatibility MR has been created (should be done by a maintainer if you're an external contributor): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3170

What are the relevant issue numbers?

Closes #18765 (closed)