ENH/DOC: remove references to np.random.RandomState in favour of np.random.Generator

[tupui]

Description

Once NumPy minimal version is set to 1.17 #5340, I propose to do a PR to remove from the documentation/code any reference to RandomState and wrong (not best practice) usage of random number generators.

We went through this in SciPy (doc part for now). More on the PR scipy/scipy#13778

Additional context

Since NumPy 1.17, the canonical way to create random numbers is:

import numpy as np
from numpy.random import default_rng
rng = default_rng()

And If a seed is to be used. User should not use a common value such as 0 or 123456. This is to prevent bias (everyone is using the same values) and ensure sufficient entropy for the generator. More details. The good practice (from NumPy's doc) is to generate a seed like this:

from numpy.random import SeedSequence
print(SeedSequence().entropy)

Then you can use the value like

rng = default_rng(my_seed_generated_and_not_common_value)