Remove ui only documentation

[duckinator]

Replaces #311. Rebased off main so it can be merged.

Here's @kevinlinxc's original description from #311:

Since the removal of ui only was merged (rubygems/rubygems.org#3084), the guides should be updated.

I remove other mentions of it and only talk about it in the place where dropdown is mentioned, explaining that it will still show up for people that have it as their set MFA level. Perhaps this bit is unnecessary but it feels like harmless transparency.

Another idea that I think we should do is link the ui only removal blog post.

[duckinator]

I split this off of #311 because I don't have access to the upstream repository.

The changes were good, they just needed to be rebased.

The error is a CI configuration problem, and is addressed by #386. The error was fixed by merging #386.