bug#66636: Move lexical-binding warning from checkdoc to byte-compiler

[Stefan Monnier]

> If specifying lexical-binding:nil in the first line is the solution
> for those who want to keep dynamically-bound code, then yes, it should
> be definitely in NEWS, and probably also in the ELisp manual.

FWIW, I think this would be a disservice to them (and to ourselves).
In 99% of the cases it's just as easy to make the code work with
`lexical-binding:t`.

We do want to allow people to silence the warning with
`lexical-binding:nil`, but we don't want to encourage it.


        Stefan