[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
* jetty/jetty.project#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 <[email protected]>

Closes apache#18601 from kiszk/SPARK-21373.

Author: kiszk

pom.xml

@@ -133,7 +133,7 @@
     <derby.version>10.12.1.1</derby.version>
     <parquet.version>1.8.2</parquet.version>
     <hive.parquet.version>1.6.0</hive.parquet.version>
-    <jetty.version>9.3.11.v20160721</jetty.version>
+    <jetty.version>9.3.20.v20170531</jetty.version>
     <javaxservlet.version>3.1.0</javaxservlet.version>
     <chill.version>0.8.0</chill.version>
     <ivy.version>2.4.0</ivy.version>