Rename the CS 'service_start_date'-related const/vars to add clarity

What does this MR do and why?

This renames the variable in the AI-Powered admin settings section to add more clarity.
We called this date GA previously, but then the term GA evolved, and what happens on Feb 15th is a service_start (or 'cut off' from implementation/fulfillment perspective)

We discussed it in Slack: (internal)

No user-facing or logic changes are expected:

• Firstly, it just renames the variables and the constant
• Secondly, the constant is in ee/app/views/admin/application_settings/_ai_powered.html.haml, which is behind the FF. The FF is disabled for %16.7: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/admin/application_settings/general.html.haml#L123
• Entire view is for SM instances only

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Related to #433255