Merge pull request #1670 from dotnet/dougbu/3.1/newer.agents.3540

[release/5.0-preview2] Update hosted and BYOC agents used in our builds

Author: dougbu

azure-pipelines.yml

@@ -290,7 +290,7 @@ stages:
 
       - job: macOS
         pool:
-          vmImage: macOS-10.13
+          vmImage: macOS-10.14
         strategy:
           matrix:
             debug: