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/seed.rs

@@ -131,7 +131,7 @@ pub async fn seed(config: &Config, db: &Database, force: bool) -> anyhow::Result
                         &github_user.login,
                         github_user.id,
                         github_user.email.as_deref(),
-                        Some(github_user.created_at),
+                        github_user.created_at,
                         None,
                     )
                     .await