Move ``util.typing.restify()`` to ``autodoc``

[AA-Turner]

restify is only ever used within sphinx/ext/autodoc. I get that this would be a breaking change, but it really should be moved there, since it has nothing to do with core Sphinx

Originally posted by @chrisjsewell in #12284 (comment)

[picnixz]

I would suggest having a deprecation period because I'm pretty sure people are patching this one (I'm one those people !)