Apple Developer Domain Association - outdated in SETUP.md

[nesro]

The docs https://github.com/ananay/apple-auth/blob/master/SETUP.md mention a need for verifying your site with .well-known/apple-developer-domain-association.txt file.

According to this comment https://developer.apple.com/forums/thread/119616?answerId=646593022#646593022 (and my experience too), it is no longer possible to verify your site with this file.

SETUP.md should be updated and this part should be removed.