Scroll Upgrades OpenVM to v1.6.0, Fixing Critical Advisories

Scroll's zkVM prover stack upgraded from OpenVM v1.4.1 to v1.6.0, deploying a new on-chain verifier contract. OpenVM v1.5.0 targets 100 bits provable security; v1.6.0 fixes four critical advisories, including GHSA-9jfx-4f4f-497j. Scroll Sepolia upgrade set for June 18, Mainnet June 25. No user action; batch finalization delayed ~1 hour.