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/context_server/src/client.rs

@@ -127,6 +127,10 @@ struct Notification<'a, T> {
 
 #[derive(Debug, Clone, Deserialize)]
 struct AnyNotification<'a> {
+    #[expect(
+        unused,
+        reason = "Part of the JSON-RPC protocol - we expect the field to be present in a valid JSON-RPC notification"
+    )]
     jsonrpc: &'a str,
     method: String,
     #[serde(default)]