[C++] C++20: use std::span

[pitrou]

Describe the enhancement requested

We currently have a std::span backport in arrow::util::span, we can now use the standard class directly and get rid of the backport.

Component(s)

C++

[chakkk309]

Hi, may I pick up this issue?

[pitrou]

Issue resolved by pull request 49492
#49492