Update cert-mananger subchart

Summary

We currently ship cert-manager 1.12, which will go EOL in May 2025.

Upgrading to a new certmanager release involves some potentially breaking changes, so it is better to do this in the next major chart release (9.0).

Acceptance Criteria

• Cert-manager subchart upgraded to supported release
• Cert-manager upgrade notes are linked from the GitLab chart 8->9 upgrade notes
• Functionality tested in fresh-install and upgrade scenario