Fix removal policy for accepting-merge-requests

I made a mistake in this MR: !565 (diffs)

As per the docs:

Note: All specified labels must be present on the resource for the condition to be satisfied

This MR fixes it by using the braces expansion syntax.

Action items

• (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 =>
  • Week-in-review