ci: Use Ubuntu 26.04 runners - #88

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: redhat-actions/buildah-build@7a95fa7ee0f02d552a32753e7414641a04307056. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/