Update mac Image to 10.14 in CI as 10.13 is going to be deprecated in March (#1182)

https://devblogs.microsoft.com/devops/removing-older-images-in-azure-pipelines-hosted-pools/

Author: bergmeister

.vsts-ci/azure-pipelines-ci.yml

@@ -52,7 +52,7 @@ jobs:
 - job: 'PS6_macOS'
   displayName: PowerShell 6 | macOS
   pool:
-    vmImage: 'macOS-10.13'
+    vmImage: 'macOS-10.14'
   steps:
   - template: templates/ci-general.yml