Update AI-powered admin settings on CS GA day

What does this MR do and why?

The goal of this MR is to prepare UI/UX changes that will happen on the "Service Start Date" (Code Suggestions will become GA).
Details: https://gitlab.com/gitlab-org/gitlab/-/issues/426374

The changes on The Date:

• The "Code Suggestion" checkbox and the section will disappear.
• The "Enable Experiment and Beta AI-powered features" toggle will not be wrapped in a section.
• Only Ultimate customers will see the "AI-powered" menu section in the Admin Area (currently it's visible to Premium as well)

Check the screenshots for more details.

This is built on top of !138337 (merged)

This MR is for Self-Managed instances only.
All changes are behind the FF (updated_ai_powered_features_menu_for_sm), and will not affect 16.7.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before the "Service Start Date"	After the "Service Start Date"

How to set up and validate locally

1. Checkout the branch: 426374-cs-to-ga-ui-changes
2. Make sure you have the Ultimate license in your GDK and run it in SM mode (GITLAB_SIMULATE_SAAS=0 gdk restart if needed)
3. In rails c, enable the FF: Feature.enable(:updated_ai_powered_features_menu_for_sm).
4. Visit http://localhost:3000/admin/application_settings/general and scroll down.
5. Make sure you see both checkboxes (Code Suggestions and Beta features)
6. Update the date in ee/lib/code_suggestions/activation.rb to somewhere in the past
7. Restart the app, reload the page (you may need to update the cache or open in a new tab), check that there is only one checkbox (Beta)
8. Alternatively, you can set the date in ee/lib/code_suggestions/activation.rb to 3-5 minutes in the future, restart the app, and check the app before and after the time (reload the page: cmd+R). Take your TZ into account!

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 #426374