Add AsciiExt is_ascii to autoharness CI - #682

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, cda-tum/setup-z3@v1.6.1, Swatinem/rust-cache@v2.7.8. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/