Require milestones on PRs before merging (#6431)

* Require milestones on PRs before merging

Signed-off-by: Sebastian Malton <sebastian@malton.name>

.github/workflows/require-milestone.yml

@@ -0,0 +1,14 @@
+name: Require Milestone
+on:
+  pull_request:
+    types: [opened, edited, synchronize]
+jobs:
+  milestone:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Require Milestone
+        run: |
+          exit $(gh pr view ${{ github.event.pull_request.number }} --json milestone | jq 'if .milestone == null then 1 else 0 end')
+        env:
+          GH_TOKEN: ${{ secrets.GH_TOKEN }}