Use non-deprecated v3 checkout runner in milestone check (#2754)

Move milestone-check.json to the right place

.github/workflows/milestone-checker.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Actions
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           repository: "grafana/grafana-github-actions"
           path: ./actions