Suggestion to add "in review" label

What does this MR do and why?

There is currently an automated instruction to add workflowin dev if the MR author needs to make changes. It seems fitting to provide instructions for the converse action to add workflowin review when the reviewer undertakes the review.

Describe in detail what your merge request does and why.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review