[Helix] Go back to 1604 for now (#22514)

1804 doesn't have enough cores to support our volume right now

Author: HaoK

eng/targets/Helix.Common.props

@@ -20,7 +20,7 @@
 
   <!-- PR(ci.yaml) required queues -->
   <ItemGroup Condition="'$(IsRequiredCheck)' == 'true' AND '$(TargetArchitecture)' == 'x64'">
-    <HelixAvailableTargetQueue Include="Ubuntu.1804.Amd64.Open" Platform="Linux" />
+    <HelixAvailableTargetQueue Include="Ubuntu.1604.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Windows.10.Amd64.Open" Platform="Windows" />
     <HelixAvailableTargetQueue Include="OSX.1014.Amd64.Open" Platform="Linux" />
   </ItemGroup>
@@ -29,7 +29,7 @@
   <ItemGroup Condition="'$(IsWindowsOnlyTest)' != 'true' AND '$(TargetArchitecture)' == 'x64' AND '$(IsHelixDaily)' == 'true'">
     <HelixAvailableTargetQueue Include="Windows.7.Amd64.Open" Platform="Windows" />
     <HelixAvailableTargetQueue Include="Windows.81.Amd64.Open" Platform="Windows" />
-    <HelixAvailableTargetQueue Include="Ubuntu.1604.Amd64.Open" Platform="Linux" />
+    <HelixAvailableTargetQueue Include="Ubuntu.1804.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Ubuntu.2004.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Centos.7.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Debian.8.Amd64.Open" Platform="Linux" />