CI: Re-enable milestone-check (#71718)

This will hopefully resolve stuck PRs until we find a better way to
disable that check.

.github/pr-checks.json

@@ -1,4 +1,11 @@
 [
+  {
+    "type": "check-milestone",
+    "title": "Milestone Check",
+    "targetUrl": "https://github.com/grafana/grafana/blob/main/contribute/merge-pull-request.md#assign-a-milestone",
+    "success": "Milestone set",
+    "failure": "Milestone not set"
+  },
   {
     "type": "check-label",
     "title": "Backport Check",