Document Go Version Upgrade Process for Gitaly

Currently, the process for upgrading Go versions in Gitaly requires following changes across multiple repositories to make sure the version has been in use for at least 3 version. As discussed in !7650 (merged), this process can be challenging for the people that are unfamiliar with the flow.

We need comprehensive documentation that outlines the complete process for upgrading Go versions in Gitaly, including:

1. Identifying all files that need to be updated (go.mod files, CI configuration, README, etc.)
2. Understanding dependencies on other repositories (like CNG)
3. Prerequisites for adding next versions in the gitaly config matrix