Revisit test slices Javadoc with JUnit Jupiter upgrade

The class-level Javadoc of our test slice annotations usually start like this:

```
Annotation that can be used in combination with {@code @RunWith(SpringRunner.class)}
```

With JUnit 5 there is no need to add a runner so we should probably revisit the intro so that it is more consistent with the fact we aim JUnit 5 to be the default moving forward.