update Ubuntu to 20.04

Author: jonsequitur

.vsts-ci.yml

@@ -115,11 +115,11 @@ stages:
         displayName: Ubuntu
         pool:
           ${{ if eq(variables['System.TeamProject'], 'public') }}:
-            name: NetCore1ESPool-Public
-            demands: ImageOverride -equals Build.Ubuntu.1804.Amd64.Open
+            name: NetCore1ESPool-Svc-Public
+            demands: ImageOverride -equals 1es-ubuntu-2004-open
           ${{ if eq(variables['System.TeamProject'], 'internal') }}:
-            name: NetCore1ESPool-Internal
-            demands: ImageOverride -equals Build.Ubuntu.1804.Amd64
+            name: NetCore1ESPool-Svc-Internal
+            demands: ImageOverride -equals 1es-ubuntu-2004
         strategy:
           matrix:
             ${{ if in(variables['Build.Reason'], 'PullRequest') }}: