[ISSUE #3014]Replace encoding using the static variable DEFAULT_CHARSET (#4776)

Author: zhenglin-charlie-li

Diff for: eventmesh-runtime/src/main/java/org/apache/eventmesh/runtime/admin/handler/EventHandler.java

@@ -197,7 +197,7 @@ void post(HttpExchange httpExchange) {
 
         try {
             String request = HttpExchangeUtils.streamToString(httpExchange.getRequestBody());
-            byte[] rawRequest = request.getBytes(StandardCharsets.UTF_8);
+            byte[] rawRequest = request.getBytes(Constants.DEFAULT_CHARSET);
             CloudEvent event = Objects.requireNonNull(EventFormatProvider
                 .getInstance()
                 .resolveFormat(JsonFormat.CONTENT_TYPE)).deserialize(rawRequest);