.github: Bump DavidAnson/markdownlint-cli2-action from 24.2.0 to 25.0.0 - #592

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: mlugg/setup-zig@d1434d08867e3ee9daa34448df10607b98908d29. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/