Update Self-hosted model's terms and conditions page

What does this MR do and why?

This MR modifies the text displayed on the Admin Get Started page for self-hosted models. It changes the button text from "Enable self-hosted models" to "Accept GitLab Testing Agreement" and adds a link to the GitLab Testing Agreement. Additionally, it updates the text to clarify that users must accept the agreement before using self-hosted models.

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.

Before	After

How to set up and validate locally

Prerequisites

You will need:

1. An active GitLab Ultimate SM Offline Cloud License.
2. An active GitLab Duo Enterprise License.
3. To start your GDK instance in self-managed mode: GITLAB_SIMULATE_SAAS=0 && gdk start
4. If you have previously enabled Self-hosted models, reset by:
   1. In the gitlab directory, please run rails c command
   2. Then enter: ::Ai::TestingTermsAcceptance.delete_all

Test it!

1. Log in as an admin

2. Visit the page Admin Area > Self-hosted models
   

3. Test the Get Started Page

   

Related to #499379 (closed)