update Ubuntu image to 20.04

Author: jonsequitur

.vsts-ci.yml

@@ -116,7 +116,7 @@ stages:
         pool:
           ${{ if eq(variables['System.TeamProject'], 'public') }}:
             name: NetCore1ESPool-Public
-            demands: ImageOverride -equals Build.Ubuntu.1804.Amd64.Open
+            demands: ImageOverride -equals Build.Ubuntu.2004.Amd64.Open
           ${{ if eq(variables['System.TeamProject'], 'internal') }}:
             name: NetCore1ESPool-Internal
             demands: ImageOverride -equals Build.Ubuntu.1804.Amd64