Enable preview versions for .NET SDK in PR Val

Author: JoeRobich

azure-pipelines-pr-validation.yml

@@ -233,8 +233,8 @@ extends:
           inputs:
             packageType: sdk
             useGlobalJson: true
+            includePreviewVersions: true
             workingDirectory: '$(Build.SourcesDirectory)'
-            allowPrerelease
 
         - task: MicroBuildSigningPlugin@4
           inputs: