fix: harden Threadmill for 1.0 qualification - #136

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, actions/setup-java@c1e323688fd81a25caa38c78aa6df2d33d3e20d9, gradle/actions/setup-gradle@ed408507eac070d1f99cc633dbcf757c94c7933a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/