chore(gha): run prcop workflow on milestone

[ozyx]

Closes #7648

Describe your changes:

Turns out there's a milestoned and demilestoned event on pull_requests that we can use to re-trigger the milestone check.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Is this a notable change that will require a special callout in the release notes? For example, will this break compatibility with existing APIs or projects that consume these plugins?

Author Checklist

• Changes address original issue?
• Tests included and/or updated with changes?
• Has this been smoke tested?
• Have you associated this PR with a type: label? Note: this is not necessarily the same as the original issue.
• Have you associated a milestone with this PR? Note: leave blank if unsure.
• Testing instructions included in associated issue OR is this a dependency/testcase change?

Reviewer Checklist

• Changes appear to address issue?
• Reviewer has tested changes by following the provided instructions?
• Changes appear not to be breaking changes?
• Appropriate automated tests included?
• Code style and in-line documentation are appropriate?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.30%. Comparing base (5b4ee19) to head (99b5b77).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7647   +/-   ##
=======================================
  Coverage   56.30%   56.30%           
=======================================
  Files         672      672           
  Lines       27120    27120           
  Branches     2632     2632           
=======================================
  Hits        15270    15270           
  Misses      11522    11522           
  Partials      328      328           

Flag	Coverage Δ
e2e-full	23.64% <ø> (ø)
e2e-stable	60.36% <ø> (ø)
unit	49.09% <ø> (ø)

see 3 files with indirect coverage changes

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