feat(ci): Validate release-notes for PRs

.github/workflows/pr-release-notes-validation.yaml

@@ -0,0 +1,36 @@
+name: Validate release-notes from PRs
+
+on:
+  pull_request:
+    types:
+    - opened
+    - edited
+    - reopened
+
+permissions:
+  pull-requests: read
+
+jobs:
+  validate-release-notes:
+    runs-on: ${{ vars.DEFAULT_RUNNER || 'ubuntu-latest' }}
+    steps:
+    - name: install-gardener-gha-libs
+      uses: gardener/cc-utils/.github/actions/install-gardener-gha-libs@master
+
+    - name: raise-on-malformed-release-notes
+      shell: python
+      run: |
+        import release_notes.model
+
+
+        pr_body_content = '''\
+        ${{ github.event.pull_request.body }}
+        '''
+
+        _, malformed = release_notes.model.iter_source_blocks(
+            source={},
+            content=pr_body_content,
+        )
+
+        if malformed:
+            raise ValueError(f'do not know how to handle malformed release-notes blocks: {malformed}')