Update Helix queue osx.1200.amd64.open to osx.15 (#5726)

* Update Helix queue osx.1200.amd64.open to osx.15

* Remove OSX.15.Arm64 as it's not ready.

Author: imcarolwang

eng/SendToHelix.proj

@@ -45,8 +45,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TestJob)' == 'MacOS'" >
-    <HelixTargetQueue Include="OSX.1200.Amd64.Open" />
-    <HelixTargetQueue Include="OSX.1200.Arm64.Open" />
+    <HelixTargetQueue Include="OSX.15.Amd64.Open" />
   </ItemGroup>
 
   <PropertyGroup Condition=" '$(RunAsPublic)' == 'true' ">