Drop git version check

What does this Merge Request do and why?

Starting with commit 7f9378de (Build and use gitaly built git, 2021-03-16), we're no longer using git provided by the developer's system but instead use git as provided by Gitaly's make git target. So while we still do require a system-provided git such that we can correctly clone and fetch repositories, we do not have any specific version requirements on its version anymore.

Drop the version checks for git -- instead, we just verify that it's installed at all.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Part of gitaly#3494 (closed)