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

[Sean Conner]

It was thus said that the Great Peter Vernigorov once stated:
> 
> Looking at latest stats on
> gemini://gemini.bortzmeyer.org/software/lupa/stats.gmi it looks like
> UTF-8 (this includes unspecified charsets which per spec default to
> UTF-8) is used by 81% of pages, US-ASCII accounts for 17%.
> 
> Given this, I suggest the spec be rephrased such that it instead
> specifies minimum acceptable values of "charset" (specifically
> us-ascii and utf-8).

  Agreed.  And looking at the stats from GUS [1], text/plain is more popular
than text/gemini (by over 2:1) and UTF-8 to US-ASCII is 54% to 46%.

  -spc

[1]	gemini://gus.guru/statistics