Disallow stubbing in factories

Problem

Stubbing in factories doesn't play well with let_it_be and lead to errors like:

 Failure/Error: allow(merge_request).to receive(:fetch_ref!)
       The use of doubles or partial doubles from rspec-mocks outside of the per-test lifecycle is not supported.

See #247111 and https://gitlab.com/gitlab-org/gitlab/-/blob/00a771a3cfc095c6904b6b13f59f3435762175ae/spec/factories/merge_requests.rb#L319-324.

let_it_be helps us to improve our test efficiency. See plan#145 (closed).

Current use

Likely, stubbing in factories is only used rarely as of today:

• spec/factories/deployments.rb
• spec/factories/commits.rb
• spec/factories/projects.rb
• spec/factories/container_repositories.rb
• spec/factories/merge_requests.rb
• spec/factories/clusters/applications/helm.rb
• spec/factories/ci/builds.rb

Proposed solution

• Document that allow is discouraged in factories - !69965 (merged)
• Suggest alternatives to stubbing in factories
• Create a 👮 RuboCop rule to ban the use of allow and expect in factories
• Remove remaining offenses