Backport-action v2.1.0

What's Changed

• Support copying milestone, assignees, and requested reviewers by @jschmid1 in #394
  • use copy_assignees to copy the assignees
  • use copy_milestone to copy the milestone
  • use copy_requested_reviewers to copy the requested reviewers

New Contributors

• @jschmid1 made their first contribution in #394

Full Changelog: v2.0.0...v2.1.0