system_check: Bump minimum required Git version to v2.33.0

What does this MR do and why?

Gitaly is about to bump the minimum required Git version to v2.33.0 to make use of several features introduced since the current miminum required version, which is v2.31.0. Adapt the system check to verify we've got a recent-enough version of Git.

Part of gitaly#3815 (closed).

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