Turn off SDL validation

Based on the discussion here: dotnet/arcade#5219
Re-enabling tracked by #20690

Author: pranavkm

.azure/pipelines/ci.yml

@@ -816,7 +816,7 @@ stages:
       publishInstallersAndChecksums: true
       # This is to enable SDL runs part of Post-Build Validation Stage
       SDLValidationParameters:
-        enable: true
+        enable: false
         continueOnError: false
         params: ' -SourceToolsList @("policheck","credscan")
         -TsaInstanceURL $(_TsaInstanceURL)