Adjust release manager page to use releases.yml

Context

The monthly release date will be moved from the 22nd to the 3rd Thursday of each month (details on &1039 (closed)). Delivery tools and processes require to be updated to support the updated release date.

The SSOT for GitLab releases was established on gitlab-com/www-gitlab-com#34176 (closed): a YML file on the handbook repo that contains all the information about the GitLab releases (releases.yml)

Proposal Adjust the release manager schedule data to releases.yml

The Release manager page needs to be updated to fetch the information from the releases.yml instead of the YML file. This will allow us to remove the release_managers.yml #19579 (closed) and concentrate all the releases information in a single file.

Implementation details

• Move the release manager information from release_managers.yml to releases.yml
• Adjust the release manager page to read the information from releases.yml

Notes:

• The release managers' permissions are not affected by this change because they are handled by a configuration file in release tools. The tooling verifies if the information from the yml file matches this configuration file and grants release managers permissions.
• The release_managers.yml page should be deleted once we have done the switchover to 16.6