Add check to ensure milestone (#2747)

* Add check to ensure milestone

Stolen from https://github.com/hashicorp/vault/pull/18406

* Update .github/workflows/milestone-checker.yml

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

* Pin grafana github actions ref

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

.github/milestone-check.json

@@ -0,0 +1,9 @@
+[
+  {
+    "type": "check-milestone",
+    "title": "Milestone Check",
+    "success": "Milestone set",
+    "failure": "Milestone not set"
+  }
+]
+