Improve NSAttributedString ctors in foundation.cs

[chamons]

During the great breaking change API scrub, one API was missed in foundation.cs.

We have a sub-optimal API in initWithURL:options:documentAttributes:error due to issues in iOS 9, which we aren't supporting anymore.

I started a patch to fix this:

https://gist.github.com/e5bf4dae26f2accdaf4b6a9de76ed206

but ran into time box issues and it started getting complicated in a mechanical PR. I decided to punt it for now, thus this issue.

[chamons]

FYI @rolfbjarne @dalexsoto

[rolfbjarne]

I had a look at the NSAttributedString API, and there is significant room for improvement. That said, it seems to be possible to do it without breaking anything, so I'm postponing to .NET 7.