Marks Windows_arm64 windows_startup_test to be unflaky (#137228)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows_arm64 windows_startup_test"
}
-->
The issue flutter/flutter#134083 has been closed, and the test has been passing for [50 consecutive runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Windows_arm64%20windows_startup_test%22).
This test can be marked as unflaky.

Author: fluttergithubbot

.ci.yaml

@@ -5510,7 +5510,6 @@ targets:
 
   - name: Windows_arm64 windows_startup_test
     recipe: devicelab/devicelab_drone
-    bringup: true # https://github.com/flutter/flutter/issues/134083
     presubmit: false
     timeout: 60
     properties: