E2E: custom rubocop rule to check for missing `requires_admin` metadata
At the moment, we mark specs that are using admin user, with requires_admin
metadata tag in order to filter out tests that isn't possible to run on production, preprod, release environments that do not have an admin user.
But we do also run tests in no-admin
mode on staging environment as additional validation before production runs. This doesn't bring actual additional functional coverage.
We should opt by removing this test run type entirely and add a custom rubocop rule as additional safeguard to not forget add :requires_admin
metadata tag in case one of the admin user related code is used.
See: gitlab-org/quality/engineering-productivity/team#174 (comment 1425174644)