chore: Prepare for Rust edition bump to 2024 (without autofix) (#27791)

Successor to #27779 - in this PR I've applied changes manually, without
futzing with if let lifetimes at all.

Release Notes:

- N/A

crates/theme/src/schema.rs

@@ -4,7 +4,7 @@ use anyhow::Result;
 use gpui::{FontStyle, FontWeight, HighlightStyle, Hsla, WindowBackgroundAppearance};
 use indexmap::IndexMap;
 use palette::FromColor;
-use schemars::gen::SchemaGenerator;
+use schemars::r#gen::SchemaGenerator;
 use schemars::schema::{Schema, SchemaObject};
 use schemars::JsonSchema;
 use serde::{Deserialize, Deserializer, Serialize};