git: Document downstream users which need update on version changes

As Gitaly is starting to become the sole distributor of the git version, there will be less sites that need updating of the git version when the minimum required version changes. No matter what, some will still need updates though, while others go away.

Next up is gitlab-org/omnibus-gitlab, which is about to be converted to use Gitaly's provided git version. It always used to have a very helpful list of places which need updating when bumping the git version though, which is also going away now. So let's adopt it into Gitaly such that it does not get lost as a checklist to see whether any location was missed.