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/user_backfiller.rs

@@ -86,7 +86,7 @@ impl UserBackfiller {
                             &user.github_login,
                             github_user.id,
                             user.email_address.as_deref(),
-                            Some(github_user.created_at),
+                            github_user.created_at,
                             initial_channel_id,
                         )
                         .await?;