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-parser/src/parse_shape_specs.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::byte_char_slices)]
+
 use crate::{lex::lex_signature, parser::parse_value, trim_quotes, TokenContents};
 use nu_protocol::{engine::StateWorkingSet, ParseError, Span, SyntaxShape, Type};