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/supermaven_api/src/supermaven_api.rs

@@ -56,7 +56,15 @@ pub struct SupermavenDownloadResponse {
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct SupermavenUser {
+    #[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"
+    )]
     id: String,
+    #[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"
+    )]
     email: String,
     api_key: String,
 }