CI: Fix service validator

ubuntu-latest has been switched to 22.04. This CI requires 20.04 because
of the dependence on Python 3.9.

.github/workflows/services-json.yml

@@ -16,7 +16,7 @@ on:
 jobs:
   schema:
     name: Schema
-    runs-on: [ubuntu-latest]
+    runs-on: [ubuntu-20.04]
     steps:
       - name: Checkout
         uses: actions/checkout@v3