Use Rust 1.75's async fn in trait instead of #[async_trait]

[dtolnay]

Announcement: https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html

[dtolnay]

Note there is no change to Cargo.toml because www-rust-lang-org does not depend directly on async-trait. This code was using rocket::async_trait via this, which puts it in scope in all modules:

www.rust-lang.org/src/main.rs

Lines 6 to 7 in e89f301

	#[macro_use]
	extern crate rocket;