fix: error[E0658]: attribute name space is experimental

[SuccinctPaul]

Motivation

• Resolve error error[E0658]: attribute name space is experimental . Error logs as bellow:

error[E0658]: `#[diagnostic]` attribute name space is experimental
  --> axum-core/src/extract/mod.rs:48:5
   |
48 |     diagnostic::on_unimplemented(
   |     ^^^^^^^^^^
   |
   = note: see issue #111996 <https://github.com/rust-lang/rust/issues/111996> for more information
   = help: add `#![feature(diagnostic_namespace)]` to the crate attributes to enable
   = note: this compiler was built on 2024-02-26; consider upgrading it if it is out of date

Solution

1. we can update toolchain to after 2024-02-26.
2. we adopt the older toolchains by add #![feature(diagnostic_namespace)] to the crate attributes to enable

Adoption will be suitable for more users. And this pr is for solution#2.

See more

• Build error with Axum 0.7.5 on vanilla project: error[E0658]: `#[diagnostic]` attribute name space is experimental #2682
• Build error with Axum 0.7.5 on vanilla project: error[E0658]: #[diagnostic] attribute name space is experimental #2681
• Tracking Issue for #![feature(diagnostic_namespace)] rust-lang/rust#111996

[jplatte]

See #2693.