Revert "MINOR: Remove -SNAPSHOT in version.py (#22066)"

This reverts commit b1ff858.

Author: FrankYang0529

tests/kafkatest/version.py

@@ -128,7 +128,7 @@ def get_version(node=None):
         return DEV_BRANCH
 
 DEV_BRANCH = KafkaVersion("dev")
-DEV_VERSION = KafkaVersion("4.2.1")
+DEV_VERSION = KafkaVersion("4.2.1-SNAPSHOT")
 
 LATEST_STABLE_TRANSACTION_VERSION = 2
 # This should match the LATEST_PRODUCTION version defined in MetadataVersion.java