Broken github-release.py needs to be fixed

[Jingru923]

In our automated release process, when a tag that starts with v20 are made, TeamCity will start the automated release process.
When I was about to make a release, it didn't happen because the tag can't be detected. This is a result of previous changes in file github-release.py. This needs to be fixed

[visr]

Specifically the current_git_branch function seems to just return HEAD.
I'm not sure why rev-parse was used here, but git describe --tags --exact-match, which is already used in that script, should be fine to check if we are on a tag starting with v20.

https://github.com/Deltares/Ribasim/blob/v2024.8.0/utils/github-release.py