ci: bump actions/setup-go from 5 to 6

ci: bump actions/setup-go from 5 to 6

Bumps actions/setup-go from 5 to 6. This upgrade includes dependency
updates that incorporate a fix for a critical vulnerability.
[Originally opened at https://github.com/git-for-windows/git/pull/5811]

- [Release notes](https://github.com/actions/setup-go/releases)
- [Commits](https://github.com/actions/setup-go/compare/v5...v6)

Originally-authored-by: dependabot[bot] <support@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>