Add redirect to avoid broken links

[elf-pavlik]

I followed a link from an MDN page and ended up with 404.

MDN is updating links to FPWD, but this repo should still add a redirect to the new location.

• update FedCM to W3C FPWD w3c/browser-specs#1486
• fix: FedCM spec_url mdn/browser-compat-data#24289