fix: clippy warning of rust 1.8.4 (#14984)

# Description

I'm on rust toolchain 1.8.4, and I can see clippy warnings that can't be
caught by the ci workflow, primarily related to lifetime params.

I think it doesn't hurt to fix those in advance.

# User-Facing Changes

# Tests + Formatting

# After Submitting

crates/nu-color-config/src/style_computer.rs

@@ -169,7 +169,7 @@ impl<'a> StyleComputer<'a> {
 
 // Because EngineState doesn't have Debug (Dec 2022),
 // this incomplete representation must be used.
-impl<'a> Debug for StyleComputer<'a> {
+impl Debug for StyleComputer<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {
         f.debug_struct("StyleComputer")
             .field("map", &self.map)