Update macOS images to 10.15

Author: JohnTitor

ci/azure.yml

@@ -89,7 +89,7 @@ jobs:
 
   - job: DockerOSX64
     pool:
-      vmImage: macos-10.14
+      vmImage: macos-10.15
     steps:
       - template: azure-install-rust.yml
       - bash: LIBC_CI=1 sh ./ci/run.sh $TARGET
@@ -151,7 +151,7 @@ jobs:
   #  dependsOn: BuildChannelsOSX
   #  continueOnError: true
   #  pool:
-  #    vmImage: macos-10.14
+  #    vmImage: macos-10.15
   #  steps:
   #    - template: azure-install-rust.yml
   #    - script: sh ci/semver.sh osx
@@ -189,7 +189,7 @@ jobs:
   - job: BuildChannelsOSX
     dependsOn: StyleAndDocs
     pool:
-      vmImage: macos-10.14
+      vmImage: macos-10.15
     steps:
       - template: azure-install-rust.yml
       - script: LIBC_CI=1 sh ./ci/build.sh