Monthly release pipeline: Implement the verify CE steps

Context

Automate the creation of monthly releases in pr... (gitlab-com/gl-infra/software-delivery&1 - closed) aims to create the active stable branch one week earlier in the release schedule to allow for thorough testing and timely backports.

To support the changes coming up in the monthly release preparation, automation needs to be in place

Implement monthly_release:initial_rc:verify_ce job

After a tag has been created, there is a manual check on the monthly release template to verify if the CE stable branch contains the right commits:

The purpose of this issue is to automate that check.

Exit criteria

• A new service should be created
• The service should be called within the monthly_release_initial_rc_day:verify_ce job (to be added by #21101 (closed))
• The job requires monthly_release_initial_rc_day:tag_rc job to be successfully executed (implemented in gitlab-org/release-tools!4143 (merged))
• Make sure the logic is under a feature flag so that it won't execute anything as part of the monthly release pipeline until we enable the FF