Backport PR #31292: CI: Updated version of macos image (#31309)

Co-authored-by: MomIsBestFriend <[email protected]>

Author: 2 people

azure-pipelines.yml

@@ -4,7 +4,7 @@ jobs:
 - template: ci/azure/posix.yml
   parameters:
     name: macOS
-    vmImage: xcode9-macos10.13
+    vmImage: macOS-10.14
 
 - template: ci/azure/posix.yml
   parameters: