Update prepare-pipelines.yml to use the new jobs-based template (Azure#32999)

Author: Konrad Jamrozik

eng/pipelines/prepare-pipelines.yml

@@ -4,7 +4,7 @@ variables:
   NugetSecurityAnalysisWarningLevel: warn
 
 extends:
-  template: /eng/common/pipelines/templates/steps/prepare-pipelines.yml
+  template: /eng/common/pipelines/templates/jobs/prepare-pipelines.yml
   parameters:
     Repository: $(Build.Repository.Name)
     Prefix: net