[spec] Limit valid encodings of text/gemini to UTF-8

[Petite Abeille]

> On Dec 29, 2020, at 09:00, Petite Abeille <petite.abeille at gmail.com> wrote:
> 
> Drop the US-ASCII holdovers. Embrace UTF-8. Move on.

If this is too much to swallow, then we have to phrase is the other way around:

"Clients MUST support US-ASCII, and SHOULD support UTF-8"

The same applies to the request URL, and text/gemini links.