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

[Petite Abeille]

> On Dec 29, 2020, at 10:03, Sean Conner <sean at conman.org> wrote:
> 
>  Per this wording, any client that receives "text/plain; charset=us-ascii"
> is allowed to just drop it on the floor and do absolutely nothing with it. 

Nonsense. A compliant client MUST support UTF-8. US-ASCII is a strict subset of UTF-8. Therefore a compliant client supports US-ASCII out-of-the-box.  Nothing more, and nothing less.

>  Why do you hate textfiles.com?

Haters gonna hate :P

Isn't it, like, 3am in your timezone? Go back to bed :)