Remove more `master` references

Continuing gitlab-com/gl-infra/delivery#1440 (closed)

Handles these cases and a few more in class and method documentation:

• ReleaseTools::Security::BatchMerger - merge_master
• ReleaseTools::Security::ImplementationIssue#merge_request_targeting_master
• ReleaseTools::Security::CherryPicker
• ReleaseTools::Security::SyncRemoteService