Update strings for Explain This Vulnerability feature

What does this MR do and why?

This MR makes the following changes to the Explain This Vulnerability feature on the vulnerability details page:

1. Added a link to the Experiment badge that links off to https://docs.gitlab.com/ee/policy/alpha-beta-support.html#experiment
2. Changed the text of the action button from "Explain this vulnerability" to "Try it out"
3. In the drawer, added the sub-title "This response is generated by AI." and increased the title size.
4. Reduced the spacing between the card title and the Experiment badge, to match the spacing in the design.

Design: #407124[design_1681921959398.png]

Before	After

How to set up and validate locally

1. Enable the openai_experimentation and explain_vulnerability feature flags.
2. Clone this project: https://gitlab.com/gitlab-org/security-products/tests/webgoat.net
3. Run a pipeline against the master branch.
4. Go to Security and Compliance -> Vulnerability report.
5. The list should only contain SAST vulnerabilities. Click on any one to get to the vulnerability details page.
6. Click on the "Explain this vulnerability" button.
7. Verify that the changes mentioned in the description have been applied.

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 #406633 (closed)