[SPARK-21373][CORE] Update Jetty to 9.3.20.v20170531

## What changes were proposed in this pull request?

This PR upgrades jetty to the latest version 9.3.20.v20170531. The version includes the fix of CVE-2017-9735.

Here are links to descriptions for CVE-2017-9735.
* https://nvd.nist.gov/vuln/detail/CVE-2017-9735
* https://github.com/eclipse/jetty.project/issues/1556

Here is [a release note](https://github.com/eclipse/jetty.project/blob/jetty-9.3.x/VERSION.txt) for the latest jetty

## How was this patch tested?

tested by existing test suites

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #18601 from kiszk/SPARK-21373.