Update azure-pipelines.yml

Ubuntu image is no longer available.

Author: blckmn

azure-pipelines.yml

@@ -94,7 +94,7 @@ stages:
 
   - job: 'Linux'
     pool:
-      vmImage: 'ubuntu-16.04'
+      vmImage: 'ubuntu-20.04'
 
     steps:
     - task: UseNode@1