Include JustJ JDK in product

This ensures users don't have to install a JDK separately. It also ensures all users have the same JDK.

Things to consider:

• We can use the Eclipse JustJ project update sites to obtain ready-to-use JDK features. This will require an upgrade of Eclipse (see #28 (closed)) and Java (see #29 (closed)).
• As part of #29 (closed) we now use JustJ during setup of a development environment. However, in that environment we don't have JDK sources available. Hopefully by adding JustJ to the target platform that will be resolved as well.
• By upgrading Tycho as part of #28 (closed), we now get the following warning during build:

  [INFO] --- tycho-p2-publisher-plugin:2.3.0:publish-osgi-ee (default-publish-osgi-ee) @ org.eclipse.escet.product ---
  [INFO] Published profile IUs: [config.a.jre.javase 11.0.0, a.jre.javase 11.0.0]
  [WARNING] No system packages found in profile nor toolchain for JavaSE-15, using current JRE system packages.
  This can cause faulty dependency resolution, consider adding a definition for a 'jdk' with id=JavaSE-15 in your toolchains.xml
  [INFO] Published profile IUs: [config.a.jre.javase 15.0.0, a.jre.javase 15.0.0]
  [WARNING] No system packages found in profile nor toolchain for JavaSE-16, using current JRE system packages.
  This can cause faulty dependency resolution, consider adding a definition for a 'jdk' with id=JavaSE-16 in your toolchains.xml
  [INFO] Published profile IUs: [config.a.jre.javase 16.0.0, a.jre.javase 16.0.0]

  Note that:
  • Removing the a.jre.javase entry from the target platform doesn't help.
  • Google only gives 3 (not that useful) hits when searching for "No system packages found in profile nor toolchain for" "using current JRE system packages".
  • The changes to Tycho that make the warning appear are explained here: https://www.eclipse.org/lists/tycho-user/msg09001.html
  • The Tycho 2.0 release notes may or may not be useful for this: https://wiki.eclipse.org/Tycho/Release_Notes/2.0
  • Maybe adding JustJ to the target platform and something from the Tycho 2.0 release notes can help.
• We should also update the upgrade instructions for Java in the developers documentation for any changes we make.
• We should update the ESCET documentation for Java version changes. In particular this includes the following files, which still mention Java 8:
  • Update download-install/index.asciidoc.
  • Update download-install/install.asciidoc.
  • Update performance/tweak-perf-settings.asciidoc.