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

[Sean Conner]

It was thus said that the Great Petite Abeille once stated:
> > 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. 

  Not it's not.  Here's the origial text:

> The spec says that "Compliant clients MUST support UTF-8-encoded text/*
> responses.  Clients MAY optionally support other encodings".

  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. 
Some here migh actually prefer that, but "text/plain; charset=us-ascii" is
also "text/plain; charset=utf-8", that is, a client *can* do something
meaningful with it, unlike "text/plain; charset=CSISOLATIN3".

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

  Why do you hate textfiles.com?

> My 2¢.

  -spc (my $.02)