fix: new clippy lints for single async blocks

[kilpatty]

error: this async expression only awaits a single future [314](https://github.com/OpenPoolProject/stratum/actions/runs/4404426106/jobs/7714018855#step:9:315) --> server/src/connection.rs:69:26 [315](https://github.com/OpenPoolProject/stratum/actions/runs/4404426106/jobs/7714018855#step:9:316) | [316](https://github.com/OpenPoolProject/stratum/actions/runs/4404426106/jobs/7714018855#step:9:317) 69 | tokio::spawn(async move { write_message(cancel_token, rx, self.writer).await }); [317](https://github.com/OpenPoolProject/stratum/actions/runs/4404426106/jobs/7714018855#step:9:318) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: you can reduce it to: write_message(cancel_token, rx, self.writer)[318](https://github.com/OpenPoolProject/stratum/actions/runs/4404426106/jobs/7714018855#step:9:319) | [319](https://github.com/OpenPoolProject/stratum/actions/runs/4404426106/jobs/7714018855#step:9:320) = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_async_block [320](https://github.com/OpenPoolProject/stratum/actions/runs/4404426106/jobs/7714018855#step:9:321) = note:-D clippy::redundant-async-blockimplied by-D warnings`
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error: this async expression only awaits a single future
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--> server/src/api/server.rs:65:41
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65 | .with_graceful_shutdown(async move { cancel_token.cancelled().await });
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: you can reduce it to: cancel_token.cancelled()
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= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_async_block
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error: could not compile stratum-server (lib) due to 2 previous errors`