Bump simple-git from 3.5.0 to 3.15.0 in /.github/actions/github

Bumps [simple-git](https://github.com/steveukx/git-js/tree/HEAD/simple-git) from 3.5.0 to 3.15.0.
- [Release notes](https://github.com/steveukx/git-js/releases)
- [Changelog](https://github.com/steveukx/git-js/blob/main/simple-git/CHANGELOG.md)
- [Commits](https://github.com/steveukx/git-js/commits/simple-git@3.15.0/simple-git)

---
updated-dependencies:
- dependency-name: simple-git
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>