renovate: Add release notes to PR footer (#15145)

This PR updates the Renovate config to place the "Release Notes" section
in the PR body footer.

Also removed the `cla-signed` label, because the CLA bot just removes
it.

Release Notes:

- N/A

renovate.json

@@ -7,6 +7,5 @@
   "major": {
     "dependencyDashboardApproval": true
   },
-  "labels": ["cla-signed"],
-  "commitBody": "Release Notes:\n\n- N/A"
+  "prFooter": "Release Notes:\n\n- N/A"
 }