Previously, JsonContentAssert returns AbstractMapAssert from
extractingJsonPathMapValue. This could lead to type safety warnings
when calling one of the assert's methods with a generic varargs
parameter such as
contains(Entry<? extends Object, ? extends Object>...).

This commit replaces the use of both AbstractMapAssert and
AbstractListAssert with MapAssert and ListAssert respectively. These
classes use final methods and @SafeVargs args to prevent the
above-described problem from occurring.

See gh-9675