Enable return-await rule

It's not enabled by recommended-type-checked despite what the documentation says

eslint.config.mjs

@@ -37,6 +37,7 @@ export default [
       "no-console": "error",
       // We have type-hinted functions that in their dummy implementations return static data
       "@typescript-eslint/require-await": "off",
+      "@typescript-eslint/return-await": "error"
     },
   },
 ];