Change default behavior of GaussianProcessRegressor.sample_y's random_state variable.

[ominusliticus]

Describe the workflow you want to enable

I think it is misleading to have the sample_y method to default to reproduce the same sample every time it is called.
The default value for random_state should be None, and the documentation sample_y should state that None ensures random behavior.

Describe your proposed solution

Please see above box

Describe alternatives you've considered, if relevant

No response

Additional context

No response

[thomasjpfan]

Thank you for opening this issue! I agree it is strange to fix the random state in sample_y. For us to change the default, we would need to go through a deprecation cycle. Are you interested in opening a PR with the change?

[ominusliticus]

Absolutely! I will get right on that!