Downgrade jetty
Downgrade jetty

- Loading mime.properties and encoding.properties fails in jdisc
  on 9.3.11, see:
  https://github.com/eclipse/jetty.project/issues/723

VESPA-4359

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa