Upgrade to jetty-9.4.20.v20190813

[jsteenbeeke]

Upgraded 9.4 images for JRE 11, JRE 8 and JRE 8 Alpine to Jetty 9.4.20.v20190813.

Jetty release notes for this version can be found at: https://github.com/eclipse/jetty.project/releases/tag/jetty-9.4.20.v20190813

Seeing as this version is not a major release in any way, I have only done a minimal startup test for each changed image. Please let me know if I missed anything.

[jsteenbeeke]

Pretty sure I didn't forget: https://github.com/appropriate/docker-jetty/pull/110/files#diff-169d673d4f5fe9cfd4c29264a0e27526R11

[jsteenbeeke]

The most recent versions for earlier releases are already up-to-date on master:

• 9.2.28.v20190418
  • https://github.com/appropriate/docker-jetty/blob/master/9.2-jre7/Dockerfile#L11
  • https://github.com/appropriate/docker-jetty/blob/master/9.2-jre8/Dockerfile#L11
• 9.3.27.v20190418
  • https://github.com/appropriate/docker-jetty/blob/master/9.3-jre8/Dockerfile#L11
  • https://github.com/appropriate/docker-jetty/blob/master/9.3-jre8/alpine/Dockerfile#L11

There are only 3 more Dockerfiles in master (9.4-jre8, 9.4-jre8/alpine and 9.4-jre11), and all three of those are updated in this Pull Request. As such, I'm not sure what exactly you think I missed.