[release/2.1] Switch to consolidated Windows build images and Ubuntu 20.04 (#29264)

Author: dougbu

build/templates/default-build.yml

@@ -71,14 +71,14 @@ jobs:
     ${{ if and(eq(parameters.poolName, ''), eq(parameters.agentOs, 'macOS')) }}:
       vmImage: macOS-11
     ${{ if and(eq(parameters.poolName, ''), eq(parameters.agentOs, 'Linux')) }}:
-      vmImage: ubuntu-18.04
+      vmImage: ubuntu-20.04
     ${{ if and(eq(parameters.poolName, ''), eq(parameters.agentOs, 'Windows')) }}:
       ${{ if eq(variables['System.TeamProject'], 'public') }}:
         name: NetCore-Svc-Public
-        demands: ImageOverride -equals Build.Server.Amd64.VS2019.Open
+        demands: ImageOverride -equals 1es-windows-2019-open
       ${{ if ne(variables['System.TeamProject'], 'public') }}:
         name: NetCore1ESPool-Svc-Internal
-        demands: ImageOverride -equals Build.Server.Amd64.VS2019
+        demands: ImageOverride -equals 1es-windows-2019
   variables:
     AgentOsName: ${{ parameters.agentOs }}
     DOTNET_HOME: $(Agent.WorkFolder)/.dotnet