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/settings.rs

@@ -11,7 +11,7 @@ use gpui::{
 };
 use refineable::Refineable;
 use schemars::{
-    gen::SchemaGenerator,
+    r#gen::SchemaGenerator,
     schema::{InstanceType, Schema, SchemaObject},
     JsonSchema,
 };