Added additional event types for milestones (#3139)

.github/workflows/validate-prs.yml

@@ -11,6 +11,8 @@ on:
       - reopened
       - synchronize
       - edited
+      - milestoned
+      - demilestoned
 
 env:
   LABELS: ${{ join( github.event.pull_request.labels.*.name, ' ' ) }}
@@ -57,4 +59,4 @@ jobs:
 
             if(errors != "") {
               core.setFailed(errors)
-            }
+            }