Upgrade to Jetty 12

We have some security vulnerabilities that are reported related with Jetty 10.

Checked and on version 16.10.X, xwiki standalone packaging was upgraded to Jetty 12.0.15 https://jira.xwiki.org/browse/XWIKI-22646

Maybe it's already safe to upgrade Jetty to version 12 on the next image release.