Migrate to Jetty 12

[AuroraLS3]

I would like to be able to..

Keep Jetty up to date.

https://jetty.org/docs/jetty/12/programming-guide/migration/11-to-12.html

The Handler API changes are quite major and impact how the plugins abstraction of requests is implemented, so this is not just a version bump.

Is your feature request related to a problem? Please describe.

Jetty 11 is no longer being developed other than backported or community provided fixes.

Jetty 12 requires Java 17 to run.
bStats is reporting ~200/4600 (4%) installations running an older Java version at the time of writing, 121 of those being Java 8 and 5.4 build 1723 which is no longer supported anyway.