Update example UDF to ES6 syntax and remove related warning (#9123)

## Description

Update example UDF to ES6 syntax and remove related warning

Google-provided Dataflow templates have supported ES6 syntax since the 2023-01-03 release (see [notes](https://cloud.google.com/dataflow/docs/release-notes#January_03_2023)).

## Checklist
- [ ] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md)
- [ ] README is updated to include [all relevant information](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#readme-file)
- [ ] **Tests** pass:   `nox -s py-3.9` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [ ] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/.github/CODEOWNERS) with the codeowners for this sample

Author: ryanmadden-google

dataflow/extensible-templates/dataflow_udf_transform.js

@@ -20,11 +20,9 @@
  * @param {string} inJson input Pub/Sub JSON message (stringified)
  */
  function process(inJson) {
-    // Nashorn engine is only ECMAScript 5.1 (ES5) compliant. Newer ES6
-    // JavaScript keywords like `let` or `const` will cause syntax errors.
-    var obj = JSON.parse(inJson);
-    var includePubsubMessage = obj.data && obj.attributes;
-    var data = includePubsubMessage ? obj.data : obj;
+    const obj = JSON.parse(inJson);
+    const includePubsubMessage = obj.data && obj.attributes;
+    const data = includePubsubMessage ? obj.data : obj;
 
     if (!data.hasOwnProperty('url')) {
       throw new Error("No url found");