Add BackfillNotificationJobsWorker to cron config

What does this MR do and

• In order to email users that are over the dashboard limit regularly this worker needs to be scheduled to re-fill jobs for the limited capacity workers. This essentially ensures the configured number of parallel workers matches what is currently running.

• Refs: https://gitlab.com/gitlab-org/gitlab/-/issues/367549

• Blocked by: (needs to get merged first) Add workers to notify over limit owners (!98438 - merged)

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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