Chore: Add PR Check action enforcing a milestone is set (#43025)

Adds a PR Check action with a rule that enforces a milestone is set on the PR. 

Ref grafana/grafana-github-actions#34
Ref grafana/grafana-github-actions#42

.github/pr-checks.json

@@ -0,0 +1,9 @@
+[
+  {
+    "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"
+  }
+]

.github/workflows/pr-checks.yml

@@ -0,0 +1,28 @@
+name: PR Checks
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+  issues:
+    types:
+      - milestoned
+      - demilestoned
+
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Actions
+        uses: actions/checkout@v2
+        with:
+          repository: "grafana/grafana-github-actions"
+          path: ./actions
+          ref: main
+      - name: Install Actions
+        run: npm install --production --prefix ./actions
+      - name: Run PR Checks
+        uses: ./actions/pr-checks
+        with:
+          token: ${{secrets.GITHUB_TOKEN}}
+          configPath: pr-checks