Update Runner Admission Controller design to align with Job Router architecture

Why is this change being made?

This MR updates the GitLab Runner Admission Controller design document to align with the newer GitLab Runner Job Router architecture. The original design was created before the job router blueprint and contained outdated approaches.

Key changes:

• Updated architecture from webhook-based to gRPC streaming connections
• Focused on security with binary accept/reject decisions (removed job mutations)
• Added runner registration and controller discovery mechanism
• Documented hierarchical processing (instance → group → project)
• Integrated with KAS Job Router for Phase 1 implementation

This ensures the admission controller design is consistent with the overall job router architecture while maintaining focus on its core security purpose.

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Commits

• Update Runner Admission Controller design to align with Job Router architecture

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