Cop idea: Enforce explicit `skip_forgery_protection` instead of `self.allow_forgery_protection = false`

[phil-wkt]

Is your feature request related to a problem? Please describe.

I've noticed code with:

self.allow_forgery_protection = false

in controllers.
It achieves the same result as skip_forgery_protection, but is lower-level and not documented in the Rails API.

Describe the solution you'd like

Cop idea: Enforce explicit skip_forgery_protection instead.

Describe alternatives you've considered

Additional context