Dedicated AI updates

What does this MR do and why?

GitLab Dedicated (which always includes an Ultimate subscription) requires you to have the Duo Pro or Enterprise add-on NOW, whereas for self-managed or GitLab.com, the requirement to have an add-on will not be required until later (likely summer, or when features go GA).

I've made these changes in the current docs. I need to now backport the changes to 16.11 and 17.0. Those versions of the docs say that Dedicated is supported, but don't mention that the add-on is required. This omission could lead the reader to believe that they have access to AI features without buying an add-on.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/458975#note_1904695877

Related to: technical-writing#1037 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).