chore: bump kona client to 1.6.0

[bxue-l2]

Fix compilation errors from the kona client v1.6.0 bump (commit 502ec68). Two upstream API changes required updates:

• KonaExecutor::new gained a receipt_builder argument — pass OpAlloyReceiptBuilder::default() as the 5th arg in crates/client/src/fp_client.rs, matching kona's own single.rs client. (Resolved the cascading
  OpReceiptBuilder trait-bound and advance_to_target errors.)
• OnlineHostBackend::with_proactive_hint renamed to with_high_level_hint — updated the call in bin/host/src/cfg.rs. (Resolved the cascading type-inference error in PreimageServer::new.)

[bxue-l2]

cherry-picked #327