Use serde 1.0.221 instead of serde_derive hackery (#38137)

serde 1.0.221 introduced serde_core into the build graph, which should
render explicitly depending on serde_derive for faster build times an
obsolote method.

Besides, I'm not even sure if that worked for us. My hunch is that at
least one of our deps would have `serde` with derive feature enabled..
and then, most of the crates using `serde_derive` explicitly were also
depending on gpui, which depended on `serde`.. thus, we wouldn't have
gained anything from explicit dep on `serde_derive`

Release Notes:

- N/A

crates/collab/src/tests/random_channel_buffer_tests.rs

@@ -5,7 +5,7 @@ use anyhow::Result;
 use async_trait::async_trait;
 use gpui::{BackgroundExecutor, SharedString, TestAppContext};
 use rand::prelude::*;
-use serde_derive::{Deserialize, Serialize};
+use serde::{Deserialize, Serialize};
 use std::{
     ops::{Deref, DerefMut, Range},
     rc::Rc,

crates/collab/src/user_backfiller.rs

@@ -157,5 +157,9 @@ impl UserBackfiller {
 struct GithubUser {
     id: i32,
     created_at: DateTime<Utc>,
+    #[expect(
+        unused,
+        reason = "This field was found to be unused with serde library bump; it's left as is due to insufficient context on PO's side, but it *may* be fine to remove"
+    )]
     name: Option<String>,
 }