Switched to `JsonSerializer` / `JsonDeserializer`

[demos/kafka/wordcount] / counter

diff --git a/counter b/counter
index fe04994..5aa0393 160000 (submodule)
--- a/counter
+++ b/counter
@@ -1 +1 @@
-Subproject commit fe049944648d9675ffe66b586b6bf860fbac696f
+Subproject commit 5aa03935c0d09c363dc2b3ddcbd5fc9aac93b8e1