Remove `nightly` feature

[al8n]

This crate is limited by some nightly features in Rust, which lead to non-zero cost abstraction.

Tracking issues for things that we can remove the nightly feature and just use stable in Rust to achieve zero-cost async fn in trait.

• Tracking Issue for async_fn_in_trait, return_position_impl_trait_in_trait rust-lang/rust#91611
• RPITIT with Send trait marker breaks borrow checker rust-lang/rust#111105

[al8n]

Closed, the async trait will be supported in Rust's 1.75.0.