[release/3.1] Update hosted and BYOC agents used in our builds

- dotnet/aspnetcore-internal#3540

Author: dougbu

azure-pipelines.yml

@@ -23,7 +23,7 @@ variables:
 - ${{ if eq(variables['System.TeamProject'], 'public') }}:
   - name: _InternalRuntimeDownloadArgs
     value: ''
-  
+
 # used for post-build phases, internal builds only
 - ${{ if and(ne(variables['System.TeamProject'], 'public'), notin(variables['Build.Reason'], 'PullRequest')) }}:
   - group: DotNet-AspNet-SDLValidation-Params
@@ -270,7 +270,7 @@ stages:
 
       - job: macOS
         pool:
-          vmImage: macOS-10.13
+          vmImage: macOS-10.14
         strategy:
           matrix:
             debug: