[SPARK-56099] Upgrade spotless-plugin to 8.4.0

### What changes were proposed in this pull request?

This PR upgrades `spotless-plugin-gradle` to 8.4.0.

### Why are the changes needed?

To keep the build tooling up to date with the latest bug fixes and improvements.
- https://github.com/diffplug/spotless/releases/tag/gradle%2F8.4.0 (2026-03-18)
  - diffplug/spotless#2892
- https://github.com/diffplug/spotless/releases/tag/gradle%2F8.3.0 (2026-03-06)
  - diffplug/spotless#2848
- https://github.com/diffplug/spotless/releases/tag/gradle%2F8.2.0 (2026-01-22)
  - diffplug/spotless#2771

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Claude Opus 4.6)

Closes #558 from dongjoon-hyun/SPARK-56099.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

gradle/libs.versions.toml

@@ -35,7 +35,7 @@ checkstyle = "13.2.0"
 pmd = "7.21.0"
 spotbugs-tool = "4.9.8"
 spotbugs-plugin = "6.4.8"
-spotless-plugin = "8.1.0"
+spotless-plugin = "8.4.0"
 
 # Packaging
 cyclonedx = "2.4.1"