Monthly release pipeline: Implement monthly_release:initial_rc:start job
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:start job
A notification should be sent to the f_upcoming_release channel when release managers start the initial_rc stage. Similar to:
The notification should be triggered by monthly_release_initial_rc:start job, to be added by #21101 (closed)

