Rails 5 fix product array method delagation by manually calling .to_a in NotificationService

What does this MR do?

Fix NotificationService for Rails 5. In Rails 5 blanket array delegation was removed, so recipients.product(alerts) fails because product is not in the explicitly delegated methods of Rails. An explicit .to_a gives the same result.

spec that was failing: ee/spec/services/ee/notification_service_spec.rb:254 see https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/85910127

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Why was this MR needed?

rails5 upgrade

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
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  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
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• End-to-end tests pass (package-and-qa manual pipeline job)

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