Move available_parallelism() into utility function

[Dandandan]

Is your feature request related to a problem or challenge?

As noted by @comphead in #13579 (review)
We can move the repeated:

available_parallelism()
             .unwrap_or(NonZero::new(1).unwrap())
             .get()

into a seperate method.

Describe the solution you'd like

Create a utility method and replace all calls to std:thread::available_parallelism with this new method.

Describe alternatives you've considered

No response

Additional context

No response

[alan910127]

take