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/gpui/src/text_system/font_fallbacks.rs

@@ -1,7 +1,7 @@
 use std::sync::Arc;
 
 use schemars::JsonSchema;
-use serde_derive::{Deserialize, Serialize};
+use serde::{Deserialize, Serialize};
 
 /// The fallback fonts that can be configured for a given font.
 /// Fallback fonts family names are stored here.