CLN: break read_json, read_msgpack API, disallow string data input

revisiting #5655 due to recent #5874, it's just not right.
most pandas read_* methods take a path/url of file like. the newish read_json/read_msgpack
also accept a (byte)string of data, and tries to guess whether it's data or a filepath.

That creates weird corner cases and is sufficently wrong IMO that it's worth
making a breaking change. Users will now have to wrap their data in BytesIO/StringIO.

not much of a problem, accept the lost convenience of `pd.read_json(j)`. But since json
usually comes from a file or url, which are supported directly I'm hoping it won't be
that disruptive.

0.14.0 ofcourse.