Upgrade Gradle Actions with basic caching

[xefensor]

Summary

Upgrades gradle/actions/setup-gradle from v4 to v6 in the build and release workflows while explicitly selecting the fully open-source basic cache provider.

This replaces and supersedes #4. The Dependabot PR could not be modified by maintainers and selected v6's proprietary Enhanced Caching provider by default.

Player or developer impact

Players are unaffected. CI and release jobs use the current Gradle action while retaining an MIT-compatible cache implementation backed by GitHub Actions cache.

Basic caching may be less storage-efficient than v6 Enhanced Caching, but it avoids accepting the separate gradle-actions-caching terms and is sufficient for this repository's workflow.

Validation

• ./gradlew build
• ./gradlew runGameTestServer, when relevant
• Focused in-game testing, when relevant
• Dedicated-server or multiplayer testing, when relevant

All 20 required GameTests passed locally. Focused gameplay and multiplayer testing are not relevant to this workflow-only change.

The workflow YAML diff was checked for whitespace and indentation. The replacement PR's GitHub Actions run will validate setup-gradle@v6 with cache-provider: basic on GitHub-hosted runners.

AI assistance

• I reviewed and understand all submitted changes, including AI-assisted output.

Codex inspected PR #4 and its failing Actions log, identified the unrelated flaky GameTest failure, reviewed Gradle's v6 cache-provider licensing, implemented the approved basic-cache configuration, and ran the local validation listed above.

Documentation and release notes

• Updated CHANGELOG.md for player-facing or notable technical changes
• Updated README or system documentation when behavior or architecture changed
• Added or updated tests where practical

No changelog, architecture documentation, or new tests are needed for this two-workflow dependency update.

Visual evidence

Not applicable; this change only affects GitHub Actions configuration.

Licensing

• I have the right to contribute every included file.
• Code and non-asset contributions are provided under LICENSE-CODE.md.
• I discussed creative-asset contributions with the developer before including them.

No creative assets are included. The v6 action is configured with its open-source basic cache provider rather than the proprietary Enhanced Caching default.