chore: Bump Rust version to 1.88 (#33439)

Goodies in this version:
- if-let chains 🎉
- Better compiler perf for Zed
(https://github.com/rust-lang/rust/pull/138522)

For more, see: https://releases.rs/docs/1.88.0/

Release Notes:

- N/A

---------

Co-authored-by: Junkui Zhang <364772080@qq.com>

crates/agent_ui/src/buffer_codegen.rs

@@ -1094,15 +1094,9 @@ mod tests {
     };
     use language_model::{LanguageModelRegistry, TokenUsage};
     use rand::prelude::*;
-    use serde::Serialize;
     use settings::SettingsStore;
     use std::{future, sync::Arc};
 
-    #[derive(Serialize)]
-    pub struct DummyCompletionRequest {
-        pub name: String,
-    }
-
     #[gpui::test(iterations = 10)]
     async fn test_transform_autoindent(cx: &mut TestAppContext, mut rng: StdRng) {
         init_test(cx);