chore: bump version of macos on azure (#9650)

* chore: bump version of macos on azure

* Update .azure-pipelines.yml

Author: SimenB

.azure-pipelines.yml

@@ -21,7 +21,7 @@ jobs:
 
   - job: macOS
     pool:
-      vmImage: macos-10.13
+      vmImage: macos-10.15
     steps:
       # This step can be removed once Mercurial gets installed on the macOS image. See https://github.com/Microsoft/azure-pipelines-image-generation/issues/604
       - script: HOMEBREW_NO_AUTO_UPDATE=1 brew install mercurial