Bump to macos-12 build image

AzDO will remove macos-11 in about two weeks: https://learn.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops&tabs=yaml#recent-updates

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

@@ -95,7 +95,7 @@ jobs:
       # See https://github.com/dotnet/arcade/blob/master/Documentation/ChoosingAMachinePool.md
       pool:
         ${{ if eq(parameters.agentOs, 'macOS') }}:
-          vmImage: macOS-11
+          vmImage: macOS-12
         ${{ if eq(parameters.agentOs, 'Linux') }}:
           ${{ if and(eq(parameters.useHostedUbuntu, true), in(variables['Build.Reason'], 'Manual', 'PullRequest', 'Schedule')) }}:
             vmImage: ubuntu-20.04
@@ -311,7 +311,7 @@ jobs:
       pool:
         ${{ if eq(parameters.agentOs, 'macOS') }}:
           name: Azure Pipelines
-          image: macOS-11
+          image: macOS-12
           os: macOS
         ${{ if eq(parameters.agentOs, 'Linux') }}:
           name:  NetCore1ESPool-Svc-Internal