Prysm v7.1.7 Adds Builder Bid Selection and SSZ

Prysm v7.1.7 released. Added external builder bid selection, per-builder max execution payments, and SSZ for proposer preferences, payload attestations, and validator balances. REST client defaults to head_v2 with legacy fallback. Fixed nil headblock startup panic. Removed top-level max_execution_payment field. Includes Gloas correctness and failover improvements.