Make json-serialize always return a unibyte string (bug#70007)

Make json-serialize always return a unibyte string (bug#70007)

The JSON format is defined as a byte sequence and will always be used as
such, so returning a multibyte string makes little sense.

* src/json.c (json_out_to_string): Remove.
(Fjson_serialize): Return unibyte string.
* test/src/json-tests.el (json-serialize/roundtrip)
(json-serialize/roundtrip-scalars, json-serialize/string):
Update tests.
* doc/lispref/text.texi (Parsing JSON): Document.
* etc/NEWS: Announce.