ci: Add a workflow to validate the Renovate config

Signed-off-by: Martin Nonnenmacher <martin.nonnenmacher@doubleopen.org>

Author: mnonnenmacher

.github/workflows/renovate-validation.yml

@@ -0,0 +1,18 @@
+name: Renovate Validation
+
+on:
+  pull_request:
+    branches:
+      - main
+  push:
+    branches:
+      - main
+
+jobs:
+  renovate-validation:
+    runs-on: ubuntu-24.04
+    steps:
+    - name: Checkout Repository
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+    - name: Validate Renovate Config
+      run: npx -y --package renovate@latest -- renovate-config-validator renovate.json