chore: Rust 2024 edition

Set the formatting style to 2021 edition to avoid disrupting existing work.
Remove when possible.

src/session.rs

@@ -54,7 +54,7 @@ unsafe fn set_cloexec(fd: RawFd) -> rustix::io::Result<()> {
     if fd == -1 {
         return Err(rustix::io::Errno::BADF);
     }
-    let fd = BorrowedFd::borrow_raw(fd);
+    let fd = unsafe { BorrowedFd::borrow_raw(fd) };
     let flags = rustix::io::fcntl_getfd(fd)?;
     rustix::io::fcntl_setfd(fd, flags | rustix::io::FdFlags::CLOEXEC)
 }