Use std::thread::available_parallelism instead of num_cpus

[liurenjie1024]

          Not a reivew comment, just a glimpse of this PR, we can replace this with `std::thread::available_parallelism()`, `num_cpus` is almost unmaintained (as it no longer needs to), and has issues that are solved by the std counterpart, e.g., this one: [std::thread::available_parallelism() return 0](https://github.com/rust-lang/rust/issues/115868).

Originally posted by @SteveLauC in #373 (comment)

[SteveLauC]

Let me take this, looks pretty straightforward:)

[liurenjie1024]

Let me take this, looks pretty straightforward:)

Thanks!