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/git_hosting_providers/src/providers/github.rs

@@ -25,22 +25,38 @@ fn pull_request_number_regex() -> &'static Regex {
 
 #[derive(Debug, Deserialize)]
 struct CommitDetails {
+    #[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"
+    )]
     commit: Commit,
     author: Option<User>,
 }
 
 #[derive(Debug, Deserialize)]
 struct Commit {
+    #[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"
+    )]
     author: Author,
 }
 
 #[derive(Debug, Deserialize)]
 struct Author {
+    #[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,
 }
 
 #[derive(Debug, Deserialize)]
 struct User {
+    #[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"
+    )]
     pub id: u64,
     pub avatar_url: String,
 }