[ZEPPELIN-5059] Update jetty to 9.4.31.v20200723
[ZEPPELIN-5059] Update jetty to 9.4.31.v20200723

### What is this PR for?
Update jetty to 9.4.31.v20200723 to get the latest security fixes.

### What type of PR is it?
- Improvement

### Todos
* [ ] - Task

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-5059

### How should this be tested?
* Travis-CI: https://travis-ci.org/github/Reamer/zeppelin/builds/729239479

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Philipp Dallig <philipp.dallig@gmail.com>

Closes #3919 from Reamer/update_jetty and squashes the following commits:

f570cab56 [Philipp Dallig] Update jetty to 9.4.31.v20200723

apache/zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,jongyoul/incubator-zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,jongyoul/incubator-zeppelin,VipinRathor/zeppelin,jongyoul/incubator-zeppelin,hammertank/zeppelin,VipinRathor/zeppelin,VipinRathor/zeppelin,joroKr21/incubator-zeppelin,apache/zeppelin,prabhjyotsingh/zeppelin,prabhjyotsingh/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,apache/incubator-zeppelin,joroKr21/incubator-zeppelin,hammertank/zeppelin,jongyoul/zeppelin,zjffdu/zeppelin,hammertank/zeppelin,joroKr21/incubator-zeppelin,fogbeam/zeppelin_mirror,zjffdu/zeppelin,jongyoul/incubator-zeppelin,joroKr21/incubator-zeppelin,joroKr21/incubator-zeppelin,VipinRathor/zeppelin,jongyoul/zeppelin,apache/zeppelin,jongyoul/zeppelin,apache/incubator-zeppelin,joroKr21/incubator-zeppelin,prabhjyotsingh/zeppelin,zjffdu/zeppelin,hammertank/zeppelin,fogbeam/zeppelin_mirror,apache/incubator-zeppelin,apache/incubator-zeppelin,prabhjyotsingh/zeppelin,fogbeam/zeppelin_mirror,hammertank/zeppelin,fogbeam/zeppelin_mirror,fogbeam/zeppelin_mirror,prabhjyotsingh/zeppelin,jongyoul/zeppelin,apache/zeppelin,zjffdu/zeppelin,apache/zeppelin,apache/zeppelin,fogbeam/zeppelin_mirror,prabhjyotsingh/zeppelin,fogbeam/zeppelin_mirror,apache/zeppelin,zjffdu/zeppelin,apache/incubator-zeppelin,jongyoul/incubator-zeppelin,VipinRathor/zeppelin,jongyoul/zeppelin,prabhjyotsingh/incubator-zeppelin,hammertank/zeppelin,apache/incubator-zeppelin,hammertank/zeppelin,zjffdu/zeppelin,jongyoul/zeppelin,prabhjyotsingh/incubator-zeppelin,prabhjyotsingh/zeppelin,joroKr21/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,jongyoul/incubator-zeppelin,jongyoul/zeppelin,VipinRathor/zeppelin