That's a standard error from Java indicating that you are using an older JVM than the Classes you are attempting to run. (the Unsupported major.minor version
numbers might be different depending on scenario, but the number you are presented indicates the version of Java you need).
I usually use the Wikipedia article on "Java Class File" to find out what the Class file major.minor version number translates to as far as a Java JVM requirement.
As for Jetty 9.3+, that requires Java 8 per its announcement email.