Replaces our GroupBy with rust's ChunkBy

[brooksprumo]

Problem

We manually implemented GroupBy because it was experimental and only available in rust-nightly. Rust 1.77.0 stabilized this feature, and it is now available in rust-stable as ChunkBy. We can replace our copy with rust std.

Summary of Changes

Replace our GroupBy with rust's ChunkBy.

[jeffwashington]

lgtm
I had to assume the implemented groupby works as we expect? I assume tests would pass if we used the built in groupby?

[brooksprumo]

The rust-stable version has basically the same test as our version did: https://doc.rust-lang.org/std/primitive.slice.html#method.chunk_by, as doc tests.

I also ran the test for the pruned banks request handler, and compared the grouped_banks_to_purge, and they were identical.

Lastly, I compared the impl in the rust source code, and it is effectively identical to our copy: https://doc.rust-lang.org/src/core/slice/iter.rs.html#3293-3334

I feel good that the rust version works as expected.