Remove charset from JSON mime type as it is invalid per RFC 7159

[tchoutri]

See
https://stackoverflow.com/questions/4990095/json-specification-and-usage-of-bom-charset-encoding

Closes #1715 #559 #849

This will enable us to query Hackage, which is overly restrictive and has not changed its behaviour since 2019 haskell/hackage-server#839.

Hackage itself is not wrong, just annoying. RFC 7159 was amended in its favour: https://www.rfc-editor.org/errata/eid5853

cc @ysangkok @alt-romes

[ysangkok]

why is it a draft?

[tchoutri]

@ysangkok I wanted to ensure CI was passing as expected before submitting for review but I already assigned you so it doesn't really make sense indeed.

[ysangkok]

tests still fail:

  1) Servant.API.ContentTypes.Servant.API.ContentTypes.handleAcceptH respects the Accept spec ordering
       Falsified (after 7 tests and 1 shrink):
         ZeroToOne 0.3
         ZeroToOne 0.4
         ZeroToOne 0.8
         0
         AcceptHeader "text/plain;charset=utf-8; q=0.8, application/json; q=0.4, application/octet-stream; q=0.3"
         "text/plain;charset=utf-8" /= "text/plain"