jextract: Support Java DoubleUnaryOperator func interface - #889

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/setup-java@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Error: Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.