[iOS]update ci 1

Author: xgfd3

cicd/build-template/build-ios.yml

@@ -9,7 +9,7 @@ jobs:
     displayName: ${{ parameters.displayName }}
 
     pool:
-      vmImage: 'macOS-10.14'
+      vmImage: 'macOS-10.15'
 
     variables:
       - group: AgoraKeys