Refactor (ci): Bump azure-pipelines vmImage versions (continued)

- Use `ubuntu-20.04` due to `ubuntu-18.04` deprecation

Author: joeltimothyoh

azure-pipelines.yml

@@ -72,7 +72,7 @@ stages:
   - job: linux_container
     displayName: '[Linux] [Container] PowerShell Core'
     pool:
-      vmImage: ubuntu-18.04
+      vmImage: ubuntu-20.04
     container: joeltimothyoh/powershell:6.1.0-ubuntu-18.04-git
     steps:
     - checkout: self