Bump GitHub Actions to Node 24-compatible versions

[richardwooding]

Summary

GitHub is deprecating Node 20 on Actions runners starting 2026-06-02 (deprecation notice). All five pinned actions in .github/workflows/ were on Node 20; this PR bumps each to the latest stable major that ships with runs.using: node24.

Action	Old	New	Node runtime
actions/checkout	@v4	@v6	node20 → node24
actions/setup-java	@v4	@v5	node20 → node24
actions/upload-artifact	@v4	@v7	node20 → node24
gradle/actions/setup-gradle	@v4	@v6	node20 → node24
softprops/action-gh-release	@v2	@v3	node20 → node24

Each new version was verified by fetching its action.yml and confirming using: node24 in the runs section. No other changes were made to the workflow files.

Notable changes in bumped versions

• actions/checkout@v6 — v5 introduced the Node 24 runtime; v6 is the current stable major.
• actions/setup-java@v5 — v5.0.0 upgraded to Node 24.
• actions/upload-artifact@v7 — v6 defaulted to Node 24; v7 is the current stable major and adds direct file upload support.
• gradle/actions/setup-gradle@v6 — v5 introduced Node 24; v6 is the current stable major (note: v6 moves Gradle caching to a separate proprietary component — caching behaviour in CI may differ slightly from v4).
• softprops/action-gh-release@v3 — v3.0.0 moved the runtime to Node 24.

Test plan

• CI passes on this PR (build matrix + integration tests)
• Spot-check that Gradle build cache behaviour is unchanged after setup-gradle@v6 bump
• Confirm release workflow still publishes correctly by watching the next tag push

References

• GitHub deprecation notice — Node 20 on Actions runners

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