[Button] Migrate to new beta colour tokens #5208

Part of #5119 (phase 1 — colour). First component migrated onto the redefined tokens from the eds-tokens beta, serving as the test case for the migration pattern.

Scope

• Switch Button (/next) onto the expanded static colours — expanded scale + step-based state names (🎛 Review & Refine Colour Token values #4742, 🧩 Re‑evaluate token naming conventions #4744), replacing dynamic colour tokens with static ones where the migration prescribes it
• Apply the new alpha-channel disabled states (🎨 Implement Alpha‑Channel approach for disabled states #4741)
• Consume tokens via the bundled next/css/variables.css entry (Add a bundled variables.css to the Tokens Studio release workflow #5201) — this doubles as validation input for ADR-0010 (Validate the bundled CSS entry in beta and accept ADR-0010 before the flip #5202)

Definition of Done

• Button uses the new beta colour tokens (no references to the old colour tokens)
• Disabled states use the alpha-channel tokens
• Visual regression checked against current Button
• Learnings captured to guide the remaining component migrations