fix: Update benchmark files for rand 0.9 API compatibility

After rebasing on PR #702, which updated rand from 0.10 to 0.9,
the benchmarks needed API adjustments:
- Use rand::Rng instead of rand::RngExt
- Use gen_range() instead of random_range()

This ensures benchmarks compile with the updated dependency.

Signed-off-by: Henrik Ma Johansson <dahankzter@gmail.com>

Author: dahankzter

glommio/benches/competing_io.rs

@@ -5,7 +5,7 @@ use glommio::{
     io::{ImmutableFile, ImmutableFileBuilder},
     Latency, LocalExecutorBuilder, Placement, Shares,
 };
-use rand::RngExt;
+use rand::Rng;
 use std::{
     cell::{Cell, RefCell},
     path::PathBuf,

glommio/benches/shared_channel.rs

@@ -1,5 +1,5 @@
 use glommio::{channels::shared_channel, enclose, prelude::*};
-use rand::RngExt;
+use rand::Rng;
 use std::{sync::mpsc::sync_channel, time::Instant};
 
 fn test_spsc(capacity: usize) {