Run release on ubuntu-18.04 VM image

Author: joeltimothyoh

azure-pipelines.yml

@@ -87,7 +87,7 @@ stages:
   - job: linux_container
     displayName: '[Linux] [Container] PowerShell Core'
     pool:
-      vmImage: ubuntu-16.04
+      vmImage: ubuntu-18.04
     container: joeltimothyoh/powershell:6.1.0-ubuntu-18.04-git
     steps:
     - checkout: self