Update group create worker to use correct time

What does this MR do and why?

• Changes Groups::CreateEventWorker to accept a new parameter created_at which is passed from the service so that the worker uses the exact time the group was created rather than the time the job was executed when creating the event.

How to set up and validate locally

• Stop sidekiq from running in GDK. gdk stop rails-background-jobs
• Create a new group. Note the time.
• Go make a cup of tea, boil an egg or sing Happy Birthday to yourself twice.
• Start sidekiq (gdk start rails-background-jobs)
• Assert that the time in the activity feed (or in the database) is the time the group was created, not when the job was executed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.