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

[Petite Abeille]

> On Dec 29, 2020, at 00:35, Sean Conner <sean at conman.org> wrote:
> 
>  I would ammend that to read "Compliant clients MUST support UTF-8 and
> US-ASCII encoded text/* reponses."

This is wholly redundant. UTF-8 is, by design, a /superset/ of US-ASCII. 

But the reverse is not true, obviously. Therefore the endless confusion.

Drop the US-ASCII holdovers. Embrace UTF-8. Move on.

My 2¢.