Moved user_dismissed_before? helper method to EE helper

What does this MR do and why?

Resolves #413422 (closed).

Moves the user_dismissed_before? helper method to EE helper, because it is only used by EE code.

Screenshots or screen recordings

No visible change

How to set up and validate locally

1. Run ee/spec/features/code_suggestions_alert_spec.rb locally or check pipeline for failures

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.

Related to #413422 (closed)