ci(mergify): only report conflicts on draft PRs with a milestone

This is an evolution of the idea in #3614 that should be closer to what we want. Not sure why I didn't think of this in the first place.

Pull-Request: #3660.

.github/mergify.yml

@@ -14,6 +14,11 @@ pull_request_rules:
     conditions:
       - conflict
       - -author=dependabot[bot]
+      - or:
+        - -draft # Don't report conflicts on regular draft.
+        - and: # Do report conflicts on draft that are scheduled for the next major release.
+          - draft
+          - milestone~=v[0-9]\.[0-9]{2}
     actions:
       comment:
         message: This pull request has merge conflicts. Could you please resolve them @{{author}}? 🙏