Fix warnings introduced in newer Rust Nightly

This does not (yet) upgrade ./rust-toolchain

The warnings:

* dead_code "field is never read"
* redundant_semicolons "unnecessary trailing semicolon"
* non_fmt_panic "panic message is not a string literal, this is no longer accepted in Rust 2021"
* unstable_name_collisions "a method with this name may be added to the standard library in the future"
* legacy_derive_helpers "derive helper attribute is used before it is introduced" https://github.com/rust-lang/rust/issues/79202

components/style/values/generics/font.rs

@@ -77,7 +77,6 @@ pub struct VariationValue<Number> {
 }
 
 /// A value both for font-variation-settings and font-feature-settings.
-#[css(comma)]
 #[derive(
     Clone,
     Debug,
@@ -90,6 +89,7 @@ pub struct VariationValue<Number> {
     ToResolvedValue,
     ToShmem,
 )]
+#[css(comma)]
 pub struct FontSettings<T>(#[css(if_empty = "normal", iterable)] pub Box<[T]>);
 
 impl<T> FontSettings<T> {