fix: non-exhaustive pattern in is_connection_open on old Rust versions

[5t0n3]

#39 broke compilation on Rust versions below 1.75 due to a match in is_connection_open(), namely:

tacacs-plus-rs/tacacs-plus/src/inner.rs

Lines 257 to 276 in 7a1164b

	Poll::Ready(ready) => match ready {
	// read of length 0 indicates an EOF, which happens when the other side closes a TCP connection
	Ok(0) => Ok(false),
	Err(e) => match e.kind() {
	// these errors indicate that the connection is closed, which is the exact
	// situation we're trying to recover from
	//
	// BrokenPipe seems to be Linux-specific (?), ConnectionReset is more general though
	// (checked TCP & read(2) man pages for MacOS/FreeBSD/Linux)
	io::ErrorKind::BrokenPipe | io::ErrorKind::ConnectionReset => Ok(false),
	// bubble up any other errors to the caller
	_ => Err(e),
	},
	// if there's data still available, the connection is still open, although
	// this shouldn't happen in the context of TACACS+
	Ok(1..) => Ok(true),
	},

The behavior of matching usizes against open ranges landed in rust-lang/rust#116692, which was first included in 1.75.0.

To keep compatibility with older versions of Rust, this PR changes the range to an _ instead, which has the same behavior and unbreaks builds on older compilers.