RetainExceptOnCreate DeletionPolicy (#2834)

https://aws.amazon.com/about-aws/whats-new/2023/07/aws-cloudformation-deletion-policies-dev-test-cycle/

Author: PatMyron

src/cfnlint/rules/resources/DeletionPolicy.py

@@ -19,7 +19,7 @@ def check_value(self, key, path, res_type):
         """Check resource names for DeletionPolicy"""
         matches = []
 
-        valid_values = ["Delete", "Retain", "Snapshot"]
+        valid_values = ["Delete", "Retain", "RetainExceptOnCreate", "Snapshot"]
 
         supported_functions = ["Fn::FindInMap", "Fn::If", "Ref"]