Enforce minimum required Git version

While documentation points out that the minimum required Git version is v2.24.0, Gitaly doesn't enforce this version anywhere. The result is that it may run in unsupported configurations, which in the worst case may lead to inconsistent behaviour or data loss.

This MR addresses that shortcoming by adding a Git version check to Gitaly's startup which enforces the minimum required version. If an older Git version is configured as Git binary, then we now refuse to start.

This is in preparation for our shift to mandatory Git v2.29.0, see #2959 (closed).