Improve UI text of Security Policy experimental features

What does this MR do and why?

Improve UI text of Security Policy experimental features:

• Subsection titles:
  • Remove "Experiment" from the title of each section as this is duplicated by the Experiment badge.
  • Change to sentence case (in accordance with GitLab design system).
• Minor editing of description of "Security policy pipeline execution action", including using "CI/CD" instead of "CI".
• Changed "Security policy scopes" checkbox title to sentence case (in accordance with GitLab design system).
• Minor editing of both features' checkboxes' descriptions:
  • Use "Enable this feature to achieve goal...", removing unnecessary "allows you to".
• Testing agreement - rephrased statement per the convention agreed on in !118222 (merged).

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

Note: In the After screenshot has 2 checkboxes selected. I forgot to clear them before taking the screenshot. They are not selected by default.

Before

After

How to set up and validate locally

Prerequisites:

• Ultimate license applied
• Group owner

1. Log in to GitLab.
2. Go to the group's home page.
3. In the left sidebar, go to Settings > General.
4. Expand Permissions and group features.
5. Scroll to the end of this section.