Change diff cover check threshold from 50 to 80 (#108)

Why I did it
Requirement is to have code diff coverage be at 80%

How I did it
Change code diff coverage threshold to 80% in azure-pipelines yml file

How to verify it
Check azure-pipelines yml file

Author: zbud-msft

azure-pipelines.yml

@@ -31,7 +31,7 @@ stages:
       vmImage: ubuntu-20.04
 
     variables:
-      DIFF_COVER_CHECK_THRESHOLD: 50
+      DIFF_COVER_CHECK_THRESHOLD: 80
       DIFF_COVER_ENABLE: 'true'
       DIFF_COVER_WORKING_DIRECTORY: $(System.DefaultWorkingDirectory)/sonic-telemetry