Bump git2 to 0.20.1

[TheVeryDarkness]

As git2 is broken on Windows since some point and git2@0.20.1 includes a fix for it, maybe the git2 version should be updated to 0.20.1.

See:

• git2-rs issue #1142.
• git2-rs pull #1143.
• libgit2 issue #7053.

Maybe other platforms, such as Windows, can also be tested via CI.

[TheVeryDarkness]

The running result can be seen here: https://github.com/TheVeryDarkness/git2version/actions/runs/18089364272/job/51466216569#logs

[TheVeryDarkness]

And I'm also working to solve this, but I'm not familiar with this repo.