chore(deps): update actions/setup-java action to v6 - #60

Dependency Update Summary

Notes

• ESM migration (PR #1078): internal packaging change. Per maintainer note in #1090, this is explicitly not a user-facing breaking change — existing uses: actions/setup-java@v6 invocations work unchanged.
• Zulu → Azul Metadata API (#1010): internal vendor API swap for resolving Zulu distributions. No action or input changes for consumers; resolved the same distributions.
• Legacy Adopt distributions removed (#1185): distributions adopt and adopt-hotspot are gone. This workflow uses temurin, so not affected.
• Input rename: jdk-file replaces jdkFile (#1083); the old name remains as a deprecated alias, so the rename is backward compatible.
• Other internal changes: cache-primary-key output added, GPG signing key handling hardened, several security patches (e.g. js-yaml audit, template-injection fixes), Maven/Gradle dependency-cache path overrides, floating-version resolution cache. All non-breaking for existing consumers.
• Compatibility check: this workflow only uses java-version and distribution: temurin — both unchanged from v5. Inputs that were renamed/removed (jdkFile legacy alias, Adopt distribution) are not used here.
• Recommendation: REVIEW — major version bump with no known breaking changes for this workflow's usage. Suggest a single CI run to confirm behavior, then close.
• No code in src/ consumes this action directly (GitHub Actions only).

📋 Tracked in #9