Refactor scripting module

Since Nashorn JavaScript Engine is deprecated in Java 11 (https://openjdk.java.net/jeps/335)
it is better do nothing with JavaScript in the project any more

* Replace JS script tests to some other languages
* Mentioned deprecation in the Docs
* Rework `scripting.adoc` for code snippet switches

spring-integration-scripting/src/test/resources/scripts/TestRouterScript.js

@@ -1,3 +0,0 @@
-(function(){
-    return payload.length > 5 ? "longStrings" : "shortStrings";
-})();

spring-integration-scripting/src/test/resources/scripts/TestRouterScript.py

@@ -0,0 +1 @@
+'longStrings' if len(payload) > 5 else 'shortStrings'