clippy: manual_div_ceil

[brooksprumo]

Problem

New clippy lints when upgrading to rust 1.84.0.

warning: manually reimplementing `div_ceil`
  --> core/benches/shredder.rs:60:22
   |
60 |     let num_shreds = ((1000 * 1000) + (shred_size - 1)) / shred_size;
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.div_ceil()`: `(1000 * 1000).div_ceil(shred_size)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_div_ceil
   = note: `#[warn(clippy::manual_div_ceil)]` on by default

Summary of Changes

Replace with div_ceil().

Similar to #4376
Partially fixes #4380

[brooksprumo]

@KirillLykov Requesting your review mostly given your general knowledge with the bench-tps and friends code. This PR is to fix a clippy lint in the new rust version, so should be quite self contained.

[KirillLykov]

a reasonable change to me, thanks for fixing