Switched to `JsonSerializer` / `JsonDeserializer`

[demos/kafka/wordcount] / counter

diff --git a/counter b/counter
index fe04994..140800c 160000 (submodule)
--- a/counter
+++ b/counter
@@ -1 +1 @@
-Subproject commit fe049944648d9675ffe66b586b6bf860fbac696f
+Subproject commit 140800c53f2422665602b8c38d2d8fe0a5d6730b