Update to mac 15

Author: wtgodbe

.azure/pipelines/jobs/default-build.yml

@@ -106,7 +106,7 @@ jobs:
       # See https://github.com/dotnet/arcade/blob/master/Documentation/ChoosingAMachinePool.md
       pool:
         ${{ if eq(parameters.agentOs, 'macOS') }}:
-          vmImage: macOS-13
+          vmImage: macOS-15
         ${{ if eq(parameters.agentOs, 'Linux') }}:
           ${{ if eq(parameters.useHostedUbuntu, true) }}:
             vmImage: ubuntu-20.04
@@ -320,7 +320,7 @@ jobs:
       pool:
         ${{ if eq(parameters.agentOs, 'macOS') }}:
           name: Azure Pipelines
-          image: macOS-13
+          image: macOS-15
           os: macOS
         ${{ if eq(parameters.agentOs, 'Linux') }}:
           name: $(DncEngInternalBuildPool)