Chore: Add auto-milestone workflow for pull-requests (#71177)

This adds a new auto-milestone workflow that will set the milestone for
a PR when it is closed. This way, contributors no longer have to think
about milestone at all.

.github/pr-checks.json

@@ -1,11 +1,4 @@
 [
-  {
-    "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",
@@ -46,4 +39,4 @@
     },
     "targetUrl": "https://github.com/grafana/grafana/blob/main/contribute/merge-pull-request.md#include-in-changelog-and-release-notes"
   }
-]
+]