Jetty12 support

[chemodanov-konstantin]

Support for Jetty 12 in Gretty is desired.
Jetty 9/10/11 community support is over and only 12 version is recommended now.

[boris-petrov]

@chemodanov-konstantin thanks for the interest! I certainly don't have the time to look into this. PRs are always welcome! One could look into these couple of PRs which were for Jetty 11. I'm not sure what the changes in Jetty 12 are but I doubt they will require as many changes - it's probably mostly going to be straight-forward changes of package versions and a couple of renamed methods.