Rollup merge of #140910 - paolobarbolini:wasi-fs-incorrect-stabilization, r=joboet

Remove `stable` attribute from wasi fs (read_exact|write_all)_at

The docs for [`std::os::wasi::fs::FileExt::read_exact_at`](https://doc.rust-lang.org/1.86.0/std/os/wasi/fs/trait.FileExt.html#method.read_exact_at) and [`std::os::wasi::fs::FileExt::write_all_at`](https://doc.rust-lang.org/1.86.0/std/os/wasi/fs/trait.FileExt.html#method.write_all_at) show the methods to be stable since v1.33, which is not correct and was a mistake made when the methods were added in (rust-lang/rust#74076 (review)). The reviewer seemed to think this was an insta-stabilization, but the entire file is marked as unstable so that was not right. The stabilization version would also have been wrong either way.

Author: matthiaskrgr