chore: add set milestone action (#9)

See [#401 - Automatically set milestone at issues when
closed](open-component-model/ocm-project#401)
for more information.

Co-authored-by: ocmbot[bot] <125909804+ocmbot[bot]@users.noreply.github.com>

.github/workflows/milestone.yaml

@@ -0,0 +1,26 @@
+name: Set milestone
+on:
+  issues:
+    types:
+      - closed
+  pull_request:
+    types:
+      - closed
+jobs:
+  set_milestone:
+    name: Set milestone
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write    
+    steps:
+      - name: Set milestone on issue
+        if: github.event.issue.state_reason == 'completed'
+        run: gh issue edit ${{ github.event.issue.number }} --milestone "$(date +"%Y-Q%q")" --repo ${{ github.repository }}
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Set milestone on pull request
+        if: github.event.pull_request.merged == true
+        run: gh pr edit ${{ github.event.pull_request.number }} --milestone "$(date +"%Y-Q%q")" --repo ${{ github.repository }}
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}