chore: migrate to Rust 2024

Removes the use of `set_var`, since it isn't thread safe.

.rustfmt.toml

@@ -1,19 +1,10 @@
-edition = "2021"
+edition = "2024"
 hard_tabs = true
-merge_derives = true
-newline_style = "Unix"
-remove_nested_parens = true
-reorder_imports = true
-reorder_modules = true
 use_field_init_shorthand = true
 # Unstable formatting options below; remove if you REALLY don't wanna use `cargo +nightly fmt`
-unstable_features = true
 format_code_in_doc_comments = true
-format_macro_bodies = true
 format_strings = true
-imports_indent = "Block"
 imports_granularity = "Crate"
 normalize_comments = true
-overflow_delimited_expr = true
 reorder_impl_items = true
 wrap_comments = true