Use JUnit 5 and remove JUnit 4 API dependencies

[pbielicki]

[pbielicki]

@marcphilipp @britter can you pls check if the direction is sensible? I primarily focused on replacing junit4 with junit5, so it's not polished but it needs significant changes in the public API, notably @Sample annotation and SamplesRunner that is being removed. It has to be replaced by other extension points. One of the ideas that I propose in this PR is to specify extra features using new @Samples annotation.

[pbielicki]

I think we should consider offering a more gradual migration path from JUnit 4 to 5. We could deprecate the Runner implementation and the old annotations for now and use the same execution logic behind the scenes. WDYT?

makes perfect sense