refactor: Deprecate #[motore::service] macro

[jexjws]

Motivation

Now that async fn in trait (AFIT) is stable in Rust, the #[motore::service] macro is no longer necessary. This macro was originally created as a workaround to provide an async fn-like developer experience by desugaring it into the fn call(...) -> impl Future signature required by the Service trait.

Since the Rust compiler can now handle this conversion automatically, we should deprecate the macro and encourage users to implement async fn call directly.

Solution

This PR deprecates the macro and removes its documentation:

1. motore-macros/src/lib.rs: Added #[deprecated] to the macro definition with a note explaining why it's no longer needed.
2. motore/src/lib.rs: Added #[deprecated] to the re-exported service macro.
3. motore/src/lib.rs: Added #[allow(deprecated)] to test_service_macro to prevent CI failures from our own deprecation warning.
4. README.md: Removed the entire section and all examples related to the #[motore::service] macro. The existing "Getting Started" example for Timeout already serves as a perfect, macro-less example of how to implement the trait.

Related

1. docs(volo/motore): add some content cloudwego.github.io#1451 (comment)

[jexjws]

emm，有必要给这两个包的小版本号加一嘛

[PureWhiteWu]

十分感谢你的贡献！
可以顺手给小版本号 +1。

[jexjws]

十分感谢你的贡献！ 可以顺手给小版本号 +1。

OK，我记住了，以后会顺手加