give botUsername a default value

Author: djamaile

changeset-feedback/action.yaml

@@ -14,7 +14,8 @@ inputs:
     required: true
   botUsername:
     description: username of the bot used to post comments on behalf of the action. Will be used when locating existing changeset feedback comments to update.
-    required: true
+    default: github-actions[bot]
+    required: false
   app-id:
     description: The Application ID of the GitHub App to use for authentication
     required: true