Explicitly declare minimum required Node.js version for any project that uses Node.js.

[krebernisak]

Neither gauntlet-solana nor gauntlet-solana-contracts give a minimum required Node.js version. The recommendation is that a minimum required Node.js version be given for any JavaScript or TypeScript project that uses Node.js.

[pythonsaber2]

Submitted #1601 with Node.js 20 engine constraints for both Gauntlet packages, aligned with the repository toolchain. Local install, build, and formatting checks pass.