Update examples for JavaScript ScriptEngine support

[sbrannen]

Overview

Several existing tests as well as examples in the reference manual refer to Nashorn as the supported ScriptEngine for JavaScript; however, Nashorn is no longer supported since Java 15.

In the interim, GraalVM has superseded Nashorn for JavaScript ScriptEngine support.

Deliverables

• Update Web MVC and WebFlux examples so that they mention GraalVM JS support instead of Nashorn.
• Create an example project (location yet to be determined) which verifies that GraalVM can be used as the JavaScript ScriptEngine for use with:
  • StandardScriptFactory
  • ScriptTemplateView
• Delete all Nashorn-specific tests which are currently disabled anyway (via @DisabledForJreRange(min = JAVA_15))