[Ban AI] Duo Availability Form for groups and admin

Context

This is part of a larger epic that gives admins more control over where AI is allowed in their organization.

Scope of this issue

This issue add UI controls to the top-level on .com and self-managed.

On self-managed: admin>settings>general>GitLab Duo (This section was partially worked on in this MR, new GitLab Duo section described in this issue)

On .com: Top-level group: settings>general>GitLab Duo (New GitLab Duo section described in this issue)

Designs

⚠ Final designs and UI copy Figma ⚠

Image	Description
	All settings options
	Selecting "off by default" shows a warning banner upon select
	Selecting "never on" shows a different warning banner upon select AND beta/experiment setting disappears
	User sees a confirmation upon save (easier to see in Figma)
	Can appear in disabled state if parent is set to never on
	Tooltip of locked setting, appears when user clicks on lock