Fix 2 NumPy deprecation warnings

[basnijholt]

Fixes:

FAILED adaptive/tests/test_learners.py::test_learner_performance_is_invariant_under_scaling[LearnerND-ring_of_fire-learner_kwargs12] - DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)

and

.venv/lib/python3.13/site-packages/adaptive/learner/learner2D.py:313: in choose_point_in_triangle
    area = 0.5 * np.cross(b - a, c - a)
        if a.shape[-1] == 2 or b.shape[-1] == 2:
            # Deprecated in NumPy 2.0, 2023-09-26
>           warnings.warn(
                "Arrays of 2-dimensional vectors are deprecated. Use arrays of "
                "3-dimensional vectors instead. (deprecated in NumPy 2.0)",
                DeprecationWarning, stacklevel=2
            )
E           DeprecationWarning: Arrays of 2-dimensional vectors are deprecated. Use arrays of 3-dimensional vectors instead. (deprecated in NumPy 2.0)