Add pipeline variables migration UI to group CI/CD settings

What does this MR do and why?

Add pipeline variables migration UI to group CI/CD settings

Introduces a new section in the Variables settings that allows group owners to disable pipeline variables in projects that don't use them. The migration changes the minimum role setting to "No one allowed" for unused projects while preserving the ability for project owners to adjust settings later.

• Add PipelineVariablesMigration Vue component
• Integrate migration section below existing Variables settings
• Include "Start migration" button with GraphQL integration
• Add loading states and toast notifications
• Follow GitLab UI patterns with proper localization keys

References

UI portion of #514242 (closed)

Screenshots or screen recordings

Screen_Recording_2025-07-30_at_11.39.53_AM

How to set up and validate locally

1. Check the GQL for this UI change in !196341 (merged)+
2. Navigate to group's CI/CD settings page and trigger the migration from the UI.
3. The projects in that group, should have the No one allowed option selected in Variables setting for the project.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #514242 (closed)