[Docs] Update GitLab upgrade documenation

Summary

Upgrades the GitLab upgrade documentation with current links and required steps, as well as links for further reading.

Notes

At the moment I've only been able to test upgrades between minor and patch versions.

All of our released Operator versions deploy GitLab 14.x.x, except for a very old 0.0.1 release which is understandably missing many features and improvements.

When we're preparing for a GitLab 15.x.x release, we can be sure to do thorough testing and update the documentation in case any special steps are required.

Closes #540 (closed)