Update the macOS Azure CI image to macos-10.15 (atom#21318)

Author: Aerijo

script/vsts/nightly-release.yml

@@ -56,7 +56,7 @@ jobs:
     timeoutInMinutes: 180
 
     pool:
-      vmImage: macos-10.14
+      vmImage: macos-10.15
 
     steps:
       - template: platforms/templates/preparation.yml

script/vsts/platforms/macos.yml

@@ -10,7 +10,7 @@ jobs:
       IsSignedZipBranch: $[ dependencies.GetReleaseVersion.outputs['Version.IsSignedZipBranch'] ]
       RunCoreMainTests: true
     pool:
-      vmImage: macos-10.14
+      vmImage: macos-10.15
 
     steps:
       - template: templates/preparation.yml
@@ -48,7 +48,7 @@ jobs:
     dependsOn: macOS_build
     timeoutInMinutes: 180
     pool:
-      vmImage: macos-10.14
+      vmImage: macos-10.15
     strategy:
       maxParallel: 3
       matrix: