Draft: Update all project dependencies and add some minor fixes and features

[HeinrichAD]

I know the PR is huge.

The changes are mainly related to external dependencies, junit tests and docker image handling.

The PR should not be merged like this! If there is an interest in these changes, I would reset the links in the readme and the GitHub workflow accordingly. Otherwise just close this PR.

---

Changes:

• update all dependencies
  • jetty 9.4 -> jetty 11
  • tomcat 9.0 -> tomcat 10
  • pom.xml artifacts
  • removed or replaced old/deprecated artifacts
  • ...
• update and fix all junit test
  • switch from httpunit (deprecated) to htmlunit
  • use java.net.URL for simple tests
• add automated junit tests with GitHub actions
• run junit tests against multiple server:
  • embedded jetty server
  • jetty docker image
  • tomcat docker image
• update checkstyle and fix errors
• fix duplicate classes and resources in classpaths
• remove http://java.sun.com/jsp/jstl/core and its dependencies and use scriptlets Normally, this could be interpreted as a step backwards. However, since the scriptlets are only used once and thus many old dependencies could be removed, this step is worthwhile.
• improve docker images
  • change BASE_URL vom build argument to environment variable
  • use -slim images to further reduce the image size
• update links to use https if possible
• add GitHub Pages

[arnaudroques]

Thanks for this PR. It contains many interesting changes. However, as you said, it's huge so we cannot merge it directly. May you break it into several smaller PR so that we can review each of them ? Thanks again.

[HeinrichAD]

I can tackle this in the next few days/weeks. However, since this is associated with some effort, I wanted to make sure that there is interest at all.

I will leave this PR open for now and link the later ones to this.

[HeinrichAD]

OK. The first three pull request are available. Let's start with these changes before we update all other dependencies (artifacts, jetty and tomcat version, etc.).

[HeinrichAD]

I think the next step is:

• jetty 9.4 -> jetty 11
• tomcat 9.0 -> tomcat 10
• fix duplicate classes and resources in classpaths

[HeinrichAD]

I thing now everything from this PR is implemented. @arnaudroques thanks for the great cooperation.

Lastly, as I was going through the open PRs, I thought: maybe it's time to close some old PRs:

• 185 - travis is not used anymore

• 113 - buildx could be extented to support more platforms if desired

  (https://docs.docker.com/buildx/working-with-buildx/#build-multi-platform-images)

• 71 - already implemented

Maybe the over 6 years old PRs should also be closed. I do not thing the changes are up to date anymore.

• 17

• 13