update signed build machine pool (#6438)

Author: brettfo

.vsts-signed.yaml

@@ -8,7 +8,7 @@ variables:
 jobs:
 - job: Full_Signed
   pool:
-    name: VSEng-MicroBuildVS2017
+    name: VSEng-MicroBuildVS2019
   timeoutInMinutes: 300
   variables:
     BuildConfiguration: 'Release'
@@ -116,5 +116,5 @@ jobs:
     dependsOn:
     - Full_Signed
     pool:
-      vmImage: vs2017-win2016
+      vmImage: windows-2019
     enablePublishBuildArtifacts: true