Make utf8 option the default for the json_pp command

[johann-petrak]

It is very odd and frustrating to see json_pp struggle when it reads a json file that has non-ascii utf8 characters in strings. This is supported by the JSON standard and hence nothing unusual really and should therefore not require a special option (which, when specified alone then also disables all the other default options, apparently).
https://datatracker.ietf.org/doc/html/rfc4627#section-3