Simplify GitHub Action entrypoint (#2119)

This commit simplifies entrypoint.sh for GitHub Actions by removing
duplication of args and black_args (cf. #1909).

The reason why #1909 uses the input id black_args is to avoid an overlap
with args, but this naming seems redundant. So let me suggest option
and src, which are consistent with CLI. Backward compatibility is
guaranteed; Users can still use black_args as well.

Commit history pre-merge:
* Simplify GitHub Action entrypoint (#1909)
* Fix prettier
* Emit a warning message when `black_args` is used

  This deprecation should be visible in GitHub Action's UI now.

Co-authored-by: Shota Ray Imaki <shota.imaki.0801@gmail.com>
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>