This commit updates the uses of math/rand to math/rand/v2, which was
released in Go 1.22 (2024-02).
The new package is generally safer, see https://go.dev/blog/randv2 for
the details.
There are no user-visible changes, it is only adjusting the name of
functions, simplify some code thanks to v2 having a better API, etc.
45580dae46