sklearn: promote 1.4-2-py312 release to preprod

Author: jinyan-li1

.github/config/image/sklearn/sagemaker-1.4-2-py312.yml

@@ -33,4 +33,4 @@ release:
   public_registry: false
   private_registry: true
   enable_soci: false
-  environment: "gamma"
+  environment: "preprod"