make JavaFixture fit for JDK 24

[akutschera]

Currently, when trying to run the tests of JavaFixture with OpenJDK 24, some tests fail because ByteBuddy fails with the following message:

Mockito cannot mock this class: class java.lang.reflect.Field.
Can not mock final classes with the following settings :
 - explicit serialization (e.g. withSettings().serializable())
 - extra interfaces (e.g. withSettings().extraInterfaces(...))

You are seeing this disclaimer because Mockito is configured to create inlined mocks.
You can learn about inline mocks and their limitations under item #39 of the Mockito class javadoc.

so we should upgrade mockito and assertj (which also includes byte-buddy as a transitive dependency) and the build in our pom in a way that will still work with Java 11 (which we use for building our artefacts) and other versions we might use when developing JavaFixture.

More or less the same thing we did when updating for Java 23.

[akutschera]

Since it's only test dependencies, I don't see a need to release a new version.

[Nylle]

I agree!