Remove `async-trait`

[masnagam]

See

• rust-lang/rust#115822
• tokio-rs/axum#2308

[masnagam]

keep using async-trait for the time being because we need dynamic dispatch. see https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html#should-i-still-use-the-async_trait-macro.