Use Ubuntu 18.04 LTS instead of 18.10 (#11045)

Author: John Luo

docs/Helix.md

@@ -3,7 +3,7 @@ Helix testing in ASP.NET Core
 
 Helix is the distributed test platform that we use to run tests.  We build a helix payload that contains the publish directory of every test project that we want to test
 send a job with with this payload to a set of queues for the various combinations of OS that we want to test
-for example: `Windows.10.Amd64.ClientRS4.VS2017.Open`, `OSX.1012.Amd64.Open`, `Ubuntu.1810.Amd64.Open`. Helix takes care of unzipping, running the job, and reporting results.
+for example: `Windows.10.Amd64.ClientRS4.VS2017.Open`, `OSX.1012.Amd64.Open`, `Ubuntu.1804.Amd64.Open`. Helix takes care of unzipping, running the job, and reporting results.
 
 For more info about helix see: [SDK](https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Helix/Sdk/Readme.md), [JobSender](https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Helix/Sdk/Readme.md)
 

eng/targets/Helix.Common.props

@@ -16,8 +16,8 @@
     <HelixAvailableTargetQueue Include="Windows.7.Amd64.Open" Platform="Windows" />
     <HelixAvailableTargetQueue Include="Windows.10.Amd64.EnterpriseRS3.ASPNET.Open" Platform="Windows" EnableByDefault="false" />
     <HelixAvailableTargetQueue Include="OSX.1013.Amd64.Open" Platform="OSX" />
-    <HelixAvailableTargetQueue Include="Ubuntu.1810.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Ubuntu.1604.Amd64.Open" Platform="Linux" />
+    <HelixAvailableTargetQueue Include="Ubuntu.1804.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Centos.7.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Debian.8.Amd64.Open" Platform="Linux" />
     <HelixAvailableTargetQueue Include="Debian.9.Amd64.Open" Platform="Linux" />