Update dependency com.diffplug.spotless:spotless-maven-plugin to v3.9.0 - #305

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, actions/setup-java@v4, joshlong/java-version-export-github-action@v28. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/