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

[Drew Adams]

> 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.

Yes, it should be documented, but not just as
a simple solution, because it's not.

FWIW: It's not a solution for code that uses
`lexical-let[*]'.  That's simply been removed
now from Emacs, unfortunately.  It's not as
simple as just setting `lexical-binding' to nil
and replacing occurrences of `lexical-binding'
with `let[*]'.

And it's not even simple to try to include the
macros for `lexical-let[*]' in your code as a
temporary stop-gap.  Ditching `lexical-let[*]'
can really break things.  Just saying...