[ci] update agent pool to ubuntu-latest.

Author: liushilongbuaa

azure-pipelines.yml

@@ -23,7 +23,7 @@ stages:
       DIFF_COVER_CHECK_THRESHOLD: 80
       DIFF_COVER_ENABLE: 'true'
     pool:
-      vmImage: ubuntu-20.04
+      vmImage: ubuntu-latest
 
     container:
       image: sonicdev-microsoft.azurecr.io:443/sonic-slave-buster:latest