Migrate to new Rust Random API

[emmanuel-ferdman]

📝 Summary

This small PR fixes a deprecation warning by updating the usage of thread_rng() to rng() from the rand crate.

💡 Motivation and Context

The rand::thread_rng() function has been deprecated in favor of the simplified rand::rng() in newer versions of the rand crate. This change aligns the code with the latest API and eliminates compiler warnings:

warning: use of deprecated function `rand::thread_rng`: renamed to `rng`
   --> crates/op-rbuilder/src/generator.rs:423:15
    |
423 |     use rand::thread_rng;
    |               ^^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

warning: use of deprecated function `rand::thread_rng`: renamed to `rng`
   --> crates/op-rbuilder/src/generator.rs:610:23
    |
610 |         let mut rng = thread_rng();
    |                       ^^^^^^^^^^

See more details here.

✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)