chore(enum-updater): fix pr close command (#35038)

### Issue # (if applicable)

Workflow job is failed(see [logs](https://github.com/aws/aws-cdk/actions/runs/16417995552/job/46488118988)), the problem is that there is an existing PR already and it tries to close it, however the confirm flag that is used to close doest not exist:

+ gh pr close 35021 --confirm
Found existing PR #35021 for module ecs, closing it...
unknown flag: --confirm
Usage:  gh pr close {<number> | <url> | <branch>} [flags]

### Reason for this change

Removed the flag and added closure description

Author: ozelalisen

.github/workflows/enum-auto-updater.yml

@@ -86,7 +86,7 @@ jobs:
             # If a PR exists, close it
             if [[ -n "$prExists" ]]; then
               echo "Found existing PR #$prExists for module ${moduleName#aws-}, closing it..."
-              gh pr close "$prExists" --confirm
+              gh pr close "$prExists" -c "Closing in favor of a new PR with updated enum values"
             else
               echo "No existing PR found for ${moduleName#aws-}"
             fi