Bump actions/setup-java from 4 to 6 - #21

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: github/codeql-action/analyze@0b1f0509b2ddb4dc7bd94367abfe7ad1d8a1b810, github/codeql-action/init@0b1f0509b2ddb4dc7bd94367abfe7ad1d8a1b810, gradle/actions/setup-gradle@42ebad84b0585498c0413b0967cf4e80ec0e1d19. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/