Update JUnit to 5.9.0

As suggested by Renovate. The reason we created a ticket is the following warning message from JUnit themselves:

Private lifecycle methods (annotated with @BeforeAll, @AfterAll, @BeforeEach, or @AfterEach) now correctly lead to an exception. Although this is a bug fix, it is technically also a breaking change since there might be existing user code with private lifecycle methods which will now start to fail.