Fix the use of generics in the Flatten Array exercise

[kahgoh]

The exercises uses <T> List<T> flatten(List<T> list). The problem with this is that T can only be an Object type because the tests provides a List of different type of objects. For example, testFiveLevelsOfNestingWithNoNulls gives a list with an int, a char and a List. Because they are different types of objects, T can only be an Object.

I propose that method's the return and parameter be updated to be either a List<Object> or List<?>.

[jagdish-15]

I'll get right onto it!

[jagdish-15]

A PR has been raised to address this issue: PR #2900. It updates the method signature in the starter code and ensures the example solution is consistent with the updated signature. Please review it at your convenience.