Revise the release process for managed versioning

What does this MR do?

Updates the GitLab Pages release process to account for managed versioning.

GitLab Pages was enrolled in managed versioning in gitlab-com/gl-infra&792 (closed).

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines