Update the module name to ReleaseCandidate and guard the VerifyStableBranchCommits logic behind an FF

What does this MR do and why?

Describe in detail what your merge request does and why.

This is a follow-up MR for gitlab-com/gl-infra/delivery#21104 (comment 2520952787) and to be in line with this MR suggestion to use ReleaseCandidate module name instead of InitialRc so that it can be reused on the final RC automation.

Related issue: gitlab-com/gl-infra/delivery#21104 (closed)

Testing

1. When early_branch_creation is disabled using the Feature class:

   bundle exec rake "monthly:initial_rc:verify_ce"
   2025-05-23 14:00:09.891131 W ReleaseTools::Feature -- Unleash::Client is disabled! Will only return default (or bootstrapped if available) results!
   2025-05-23 14:00:09.891598 W ReleaseTools::Feature -- Unleash::Client is disabled! Metrics and MetricsReporter are also disabled!

2. When early_branch_creation is enabled using the Feature class:

   Code change

   def self.enabled?(feature)
         logger.trace(__method__, feature: feature)
   
         return true if feature == :early_branch_creation
   
         UNLEASH.is_enabled?(feature.to_s)
       end

   bundle exec rake "monthly:initial_rc:verify_ce"
   2025-05-23 14:01:41.305019 W ReleaseTools::Feature -- Unleash::Client is disabled! Will only return default (or bootstrapped if available) results!
   2025-05-23 14:01:41.305435 W ReleaseTools::Feature -- Unleash::Client is disabled! Metrics and MetricsReporter are also disabled!
   2025-05-23 14:01:42.144995 I [dry-run] ReleaseTools::Monthly::ReleaseCandidate::VerifyStableBranchCommits -- Verifying CE stable branch commits -- {version: "18.1.0", stable_branch: "18-1-stable"}
   rake aborted!
   2025-05-23 14:01:42.703464 D [dry-run] ReleaseTools::GitlabClient -- [HTTParty] [2025-05-23 14:01:42 +0800] 200 "GET https://gitlab.com/api/v4/projects/gitlab-org%2Fsecurity%2Fgitlab-foss/repository/commits" 2 
   2025-05-23 14:01:42.704007 E [dry-run] ReleaseTools::Monthly::ReleaseCandidate::VerifyStableBranchCommits -- CE stable branch verification failed -- {error: "Could not find 'Update VERSION files' commit on 18-1-stable"}
   2025-05-23 14:01:42.704288 I [dry-run] ReleaseTools::Slack::ReleaseJobEndNotifier -- Posting slack message -- {job_type: "Verify CE stable branch commits", status: :failed}

Content

• Update the module name to ReleaseCandidate and guard the VerifyStableBranchCommits logic with an FF

Author Check-list

• Has documentation been updated?