GitLab upgrade notes

Upgrade notes for supported GitLab versions.

• GitLab 18 upgrade notes
• GitLab 17 upgrade notes
• GitLab 16 upgrade notes
• GitLab 15 upgrade notes
• GitLab Helm chart 9.0 upgrade notes
• GitLab Helm chart 8.0 upgrade notes
• GitLab Helm chart 7.0 upgrade notes
• GitLab Helm chart 6.0 upgrade notes