refactor: make ERC1271Forwarder.isValidSignature overridable - #61

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, foundry-rs/foundry-toolchain@82dee4ba654bd2146511f85f0d013af94670c4de. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/