Switch `experiment` badge to `beta`

Why are we doing this work

This issue tracks updating the badge from Experiment to Beta

Note that we are still waiting for definition Feature namespace toggles (&10537 - closed) to understand if we need to update what toggles we are respecting. That will be captured in another issue.

to

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

1. Update badge to Beta
2. Revise language with "is a beta feature"
3. Update feedback issue link to #413553 (closed)

Verification steps

1. Navigate to a vulnerability where ETV is enabled - https://gitlab.com/gitlab-org/govern/threat-insights-demos/personal-test-projects/webgoat.net/-/security/vulnerabilities/82203662
2. The ETV banner should have
   1. Updated badge showing Beta
   2. Revised language with "is a beta feature"
   3. Feedback issue link points to 📣 Feedback issue for: [Beta] Explain Vulnerabil... (#413553 - closed)