Jetty 12.1.x various updates to demos

[janbartel]

• Use standard spec api jars for jetty demos
• Remove some printlns from demos
• Remove use of jetty util class StringUtil

[janbartel]

@olamy the remaining uses of jetty-util classes are deliberately testing the ability of the webappclassloader to see jetty classes that are in WEB-INF/lib, or in the case of the async-rest demo, it's using the jetty-util-ajax utils to parse json.

[janbartel]

@olamy nudge?

[olamy]

LGTM
Thanks