test: Re-add 3.9.4 to integration tests

Author: Techassi

tests/test-definition.yaml

@@ -2,6 +2,7 @@
 dimensions:
   - name: zookeeper
     values:
+      - 3.9.4
       - 3.9.5
       # To use a custom image, add a comma and the full name after the product version
       # - x.x.x,oci.stackable.tech/sdp/zookeeper:x.x.x-stackable0.0.0-dev