Use uplatex for default latex_engine for Japanese docs

[tk0miya]

Is your feature request related to a problem? Please describe.
Use uplatex for default latex_engine for Japanese docs.

Describe the solution you'd like
Since v2.3, Sphinx supports uplatex as an alternative of latex_engine for Japanese docs (refs: #4186, #6841). uplatex is able to build a document without conversion character encoding internally. It allows using unicode characters in documents. Additionally, uplatex is compatible with platex (current default latex_engine for Japanese docs).

Describe alternatives you've considered
Nothing.

Additional context
Nothing.