Update azure macos builds to 10.15

Closes GH-5908.

Author: nikic

azure/macos/job.yml

@@ -5,7 +5,7 @@ parameters:
 jobs:
   - job: ${{ parameters.configurationName }}
     pool:
-      vmImage: 'macOS-10.14'
+      vmImage: 'macOS-10.15'
     steps:
     - template: brew.yml
     - script: |