Makefile: Update default Git version to v2.44
In cd08be69 (featureflag: Remove the gitaly_git_v244
flag,
2024-04-25), we have removed the feature flag for the Git v2.44 rollout.
But while it is the default Git version when using bundled Git, it is
not yet the default version when installing the Git distribution.
Update the Git distribution to use Git v2.44 and remove the now-unsused Git v2.43.