[rdf4j-dev] Github CI changes

[Håvard Ottestad <hmottestad@xxxxxxxxx>]

Hi,

I’ve added a jdk 8 and jdk 11 matrix builds (it runs everything in both version) to the Github CI files (they call it Actions).

I’ve also added two pre-steps. 

1. Run formatter:validate to check that the files were formatted correctly. 
2. Run mvn install without tests to verify that the code compiles and builds.

This makes it easier to diagnose typical issues like forgetting to format or having a compile error. 

Doesn’t increase the CI time. Also the matrix builds run in parallel :)

Håvard