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

[Petite Abeille]

> On Dec 29, 2020, at 22:24, Peter Vernigorov <pitr.vern at gmail.com> wrote:
> 
> Back to a more productive topic, the wording in the spec - "clients
> MUST support UTF-8 encoded responses" - is ambiguous and doesn't
> actually mean that acceptable value for "charset" must include
> "utf-8", and says nothing about what values of "charset" are
> acceptable.

Confused indeed.

Are you making a distinction between UTF-8 the encoding vs. utf-8 the charset? Is there such difference? What would that difference be? I feel out of depth.

But ok, if that helps in term of clarity of purpose, then more power to the spec by spelling it out.