Add GitHub Actions Maven build workflow

[AndrzejProchyra]

• added standard "Java with Maven" workflow using GitHub UI
• changed JDK version to 8
• removed optional dependency graph upload, for simplicity

The build output is nice and clean, apart from the warning that the GettingItWorking submodule does not include production code, so the JAR created by the package phase is empty.

Possible improvements:

• Additionally build on latest LTS Java (currently 21) to test the experience of workshop attendees building on modern Java (using the "matrix" functionality - https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#jobsjob_idstrategymatrix)