Refactor Mock-Utils to use new Java, JUnit5 and Mockito5 features

[wolf-bubenik]

• Update code to use latest Mockito 5
• Update code to use latest JUnit5
• Check if we still need the ThreadLocal in RepositoryMockUtils - is Mockito5 supporting multithreaded testing properly? I guess, yes.
• Check if it would make sense to provide a MagnoliaMockitoExtension
• • setting Strictness.WARN (if needed)
• • calling clearComponentProvider() after each test method
• • using MockUtils to build instance mocks or method parameter mocks
• Use Optional and stream().forEach()
• I'm sure, there is more to improve