Resolve clippy lint in peer ranking sort

[Copilot]

CI was failing on a clippy::unnecessary_sort_by lint in the epaxos peers ranking code. This update removes the lint by using the key-based sort API.

• Peer rank sorting
  • Swap the comparator-based sort with a key-based sort for the same ordering.

fn sort_rank(rank: &mut [(u64, ReplicaId)]) {
    rank.sort_unstable_by_key(|lhs| lhs.0);
}

Original prompt

fix CI failure

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[Copilot]

Pull request overview

Updates EPaxos peer ranking to satisfy clippy::unnecessary_sort_by by switching from a comparator-based sort to a key-based sort, preserving the same ordering behavior.

Changes:

• Replace sort_unstable_by(|lhs, rhs| ...) with sort_unstable_by_key(|lhs| ...) in peer rank sorting.

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