udpate rust toolchain to rust 1.81.0 (#14473)

# Description

With the release of rust 1.83.0 it's time to update to rust 1.81.0.

crates/nu-cli/src/util.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::byte_char_slices)]
+
 use nu_cmd_base::hook::eval_hook;
 use nu_engine::{eval_block, eval_block_with_early_return};
 use nu_parser::{lex, parse, unescape_unquote_string, Token, TokenContents};