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

[Alex // nytpu]

> looking at the stats from GUS [1], text/plain is more popular than
> text/gemini (by over 2:1)
If I remember correctly, most of that plaintext is from an RFC mirror
somewhere, I'm sure the actual stats of stuff written and intended for
gemini, not including mirrored content, differs significantly from GUS'
report.

-- 
Alex // nytpu
alex at nytpu.com
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