Rename `Set up Git user` step to `Set up Git`

The step no longer only configures the committer identity, it also
exports `GIT_AUTHOR_DATE`.

Assisted-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A4jy56QVeduMJcpkryJQpG