JUnit 6.0.0 is out and we can consider updating to it.

Release Notes: https://docs.junit.org/current/release-notes/index.html#release-notes-6.0.0

Notes:
- Java 17 is the baseline (so not a problem for us, since we have a higher baseline)
- Deprecation were removed (not a problem, as we don't rely on any)
- JUnit Vintage Engine is deprecated (not a problem, as we dropped support for it some months ago)

JUnit 6 now uses a single version number for all dependencies, that is platform, jupiter and vintage (which we do not use anymore).