collab: Require github_user_created_at at ingress points (#17180)

This PR makes the `github_user_created_at` field required at ingress
points into collab.

In practice we already have this value passed up, this change just makes
that explicit.

This is a precursor to making it required in the database.

Release Notes:

- N/A

crates/collab/src/api.rs

@@ -111,7 +111,7 @@ struct AuthenticatedUserParams {
     github_user_id: i32,
     github_login: String,
     github_email: Option<String>,
-    github_user_created_at: Option<chrono::DateTime<chrono::Utc>>,
+    github_user_created_at: chrono::DateTime<chrono::Utc>,
 }
 
 #[derive(Debug, Serialize)]