New style steps should use `run` instead of `start`. Convert ValidateCommitterAndReviewer to a new-style step.
Pull request: https://github.com/WebKit/WebKit/pull/8861
<rdar://problem/104599997>
Committed 259280@main (4f875349aec3): <https://commits.webkit.org/259280@main> Reviewed commits have been landed. Closing PR #8861 and removing active labels.