Create fewer backtrace frames when using #[instrument]

[TethysSvensson]

This is my attempt at fixing #3453. It does require bumping the tracing-attributes crate to rust 2024, which then requires bumping the MSRV to 1.85, but this is well within the tracing MSRV policy, which states that only the current stable and the previous 3 versions must be supported.

I have only updated the tracing-attributes crate to 2024. I also haven't updated the README or anything else that might need upgrading. If this looks interesting to you, let me know and I can look into porting the rest as well.

Implementation notes

This implements the suggestion from #3453: It stops using async fn in the generated code to avoid extra entries in the backtrace.

The reason why rust 2024 is needed, is because the lifetime rules of 2018 -- otherwise the impl Future did not infer the correct lifetime bounds.

One possible downside of this way of implementing it is that does make one of the error messages from the ui test worse. If that is not something that is acceptable, perhaps the codegen can be tweaked further to keep the original async fn and call that instead of using an async move block.