Bump minimum required Git version to v2.28.0

Patrick Steinhardtrequested to merge
pks-git-v2.28.0 into master

What does this Merge Request do?

This MR upgrades the minimum required Git version to v2.28.0. Part of gitaly#2959 (closed)

Merge Request checklist

  • Tests added for new functionality. If not, please raise Issue to follow-up.
  • This change is backward compatible. If not, please include steps to communicate to our users.
  • Documentation added/updated, if needed.
  • gdk doctor test added, if needed.

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