string.c: follow an ASCII run only as far as the index asks - #7185

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: ls-lint/action@02e380fe8733d499cbfc9e22276de5085508a5bd. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/