format-json, test: Don't force ASCII

Currently, due to ensure_ascii=True default of json.dumps, we are
insisting on our JSON schemas to be ascii-only and all other characters
to be represented by \uxxxx escapes. So far this was not problematic as
all our schemas contains only ASCII characters, but upcoming
ors-amarisoft changes want to use e.g. "→" symbol:

    https://lab.nexedi.com/kirr/slapos/blob/b51f5523/software/ors-amarisoft/software.cfg.json#L15

which currently results in failure of json-schema test:

    FAIL: test_ors-amarisoft_software_cfg_json_format (slapos.test.test_json_schema.TestJSONSchemaValidation)
    ...

    First differing element 14:
    '      "title": "\\u2192  eNB/gNB | Radio Unit",'
    '      "title": "→  eNB/gNB | Radio Unit",'

And in general, in 2023 I think there is no reason to insist on our schemas to
be ASCII-only: say if one wants to describe something about "α" parameter. It
would be good to use that α character directly and seeing it in the editor,
instead of using escapes all the time.

As indicated by below stackoverflow answer "JSON spec requires UTF-8 support by
decoders": https://stackoverflow.com/a/594881/9456786 , and indeed checking
JSON specification also confirms that by default JSON decoders shall use UTF-8:

    https://datatracker.ietf.org/doc/html/rfc7159#section-8.1

This way, I think, we can switch to UTF-8 safely.

/reviewed-by @jerome, @lu.xu
/reviewed-on !1498