Mark plugin tests as stable (#14760)

dev/devicelab/manifest.yaml

@@ -242,7 +242,6 @@ tasks:
       Checks that the project template works and supports plugins.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   flutter_gallery_instrumentation_test:
     description: >
@@ -265,7 +264,6 @@ tasks:
       Checks that the project template works and supports plugins on iOS.
     stage: devicelab_ios
     required_agent_capabilities: ["mac/ios"]
-    flaky: true
 
   external_ui_integration_test_ios:
     description: >
@@ -366,7 +364,6 @@ tasks:
       Checks that the project template works and supports plugins on Windows.
     stage: devicelab_win
     required_agent_capabilities: ["windows/android"]
-    flaky: true
 
   hot_mode_dev_cycle_win__benchmark:
     description: >