collab: Fix GitHub user retrieval in seed script (#18296)

This PR fixes the GitHub user retrieval in the database seed script.

The users returned from the [list
users](https://docs.github.com/en/rest/users/users?apiVersion=2022-11-28#list-users)
endpoint don't have a `created_at` timestamp, so we need to fetch them
individually.

I want to rework this further at a later date, this is just a bandaid to
get things working again.

Release Notes:

- N/A