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/theme/src/styles/accents.rs

@@ -1,5 +1,5 @@
 use gpui::Hsla;
-use serde_derive::Deserialize;
+use serde::Deserialize;
 
 use crate::{
     AccentContent, amber, blue, cyan, gold, grass, indigo, iris, jade, lime, orange, pink, purple,

crates/theme/src/styles/players.rs

@@ -1,7 +1,7 @@
 #![allow(missing_docs)]
 
 use gpui::Hsla;
-use serde_derive::Deserialize;
+use serde::Deserialize;
 
 use crate::{
     PlayerColorContent, amber, blue, jade, lime, orange, pink, purple, red, try_parse_color,