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

[Petite Abeille]

> On Jan 3, 2021, at 17:11, C?me Chilliet <come at chilliet.eu> wrote:
> 
> No, the specification specifies that default is utf-8, so unspecified is utf-8.

Precisely my point. Thanks.

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