fix(ci): prepare-pipelines.xml is broken due to renamed property (Azure#38233)

`NugetSecurityAnalysisWarningLevel` is renamed to `nugetMultiFeedWarnLevel`, so we need to follow up the change

Author: archerzz

eng/pipelines/prepare-pipelines.yml

@@ -1,7 +1,7 @@
 trigger: none
 
 variables:
-  NugetSecurityAnalysisWarningLevel: warn
+  nugetMultiFeedWarnLevel: warn
 
 extends:
   template: /eng/common/pipelines/templates/jobs/prepare-pipelines.yml