Chore: Add PR Check action enforcing backport decision (#43916)

Ref https://github.com/grafana/grafana-github-actions/pull/53
Ref https://github.com/grafana/grafana-release/issues/60

.github/pr-checks.json

@@ -5,5 +5,14 @@
     "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-backport",
+    "title": "Backport Check",
+    "backportEnabled": "Backport enabled",
+    "backportSkipped": "Backport skipped",
+    "failure": "Backport decision needed",
+    "targetUrl": "https://github.com/grafana/grafana/blob/main/contribute/merge-pull-request.md#should-the-pull-request-be-backported",
+    "skipLabels": [ "backport", "no-backport"]
   }
 ]

.github/workflows/pr-checks.yml

@@ -4,6 +4,8 @@ on:
     types:
       - opened
       - synchronize
+      - labeled
+      - unlabeled
   issues:
     types:
       - milestoned