Update copy on Self-hosted AI models form

What does this MR do and why?

Implements the follow-up copy updates as discussed here

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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

Self-hosted models enabled

Before	After

Self-hosted models not enabled

Before	After

How to set up and validate locally

Prequisites

You will need an active GitLab ultimate license.

1. Start GDK instance in self-managed mode:

GITLAB_SIMULATE_SAAS=0 && gdk start

2. In the rails console, enable Code Suggestions and the custom models experiment:

Feature.enable(:ai_custom_model)
ApplicationSetting.first.update(duo_features_enabled: true)

3. Navigate to the GitLab Duo seat assignment page. You should a page that looks like the below:

If you don't, run the following rake task to complete Gitlab Duo setup

bundle exec rake gitlab:duo:setup

Test it!

1. Log in as an admin
2. Visit the page Admin Area > Settings > General\
3. You should see an expandable section called "Gitlab Duo features" - expand it
4. You should see a sub-section called "Self-hosted AI models" - this is the added form. Once the terms and conditions have been accepted, the intention is that it cannot be reversed by the user. To reset the form's state locally, follow directions below.

Reset form state:

1. Open your rails console rails c
2. Remove accepted terms from the database ::Ai::TestingTermsAcceptance.delete_all.

Related to #498285 (closed)