[suppressions] Add label SuppressionReviewRequired if any changes to suppressions.yaml

- [ ] Any changes to suppressions.yaml should cause label `SuppressionReviewRequired` to be added, which will block the PR until label `Approved-Suppression` is applied.
- [ ] For ARM, the ARM Reviewer will add this label, as they already do for suppressions in readme.md
- [ ] For data-plane, the reviewer is TBD
- [ ] Update text for `SuppressionReviewRequired` in NextStepsToMerge, to include suppressions.yaml and perhaps link to more/better docs
- [ ] Update docs for TypeSpecRequirement suppressions, to clarify when the suppression should and should not be applied