This is a Java project. There's no need for it to pre-cache sbt

This commit updates the Dockerfile to use JDK21 and adds configuration
to auto-download a JDK.

The Dockerfile needs to be updated to JDK21 because the autodownload
cannot find a JDK21 for Ubuntu on M1 Macs (so needs to use one set
up in the Docker image) for tutorial 3

Gradle 8 and Java 21 are the default versions installed on turing. It's
hopefully going to be most straightforward if we use these versions.

This switches to just using Swing and no modules for better compatibility with working with VS Code.

0.0.10 seems to have fixed the issue with running modular JavaFX
  projects, allowing us to drop the previous (manual) fix.