Move towards `np.random.default_rng`

[sappelhoff]

We are still using numpy's old random API in many places. It would be good to migrate to the recommended np.random.default_rng.

This might turn out to be difficult, given our many tests that need to remain compatible with matlab, etc.

xrefs:

• https://github.com/a-hurst/matprep_artifacts/tree/master
• all random code should use default_rng mne-tools/mne-python#12780

cc @a-hurst