Update plugin com.diffplug.spotless to v8

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
com.diffplug.spotless	6.25.0 → 8.7.0

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Configuration

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[iht]

Code Review Summary

PR Changes: Updates com.diffplug.spotless plugin version from 6.12.0 to 8.7.0 in build.gradle.

Build Status & Issues

The remote build workflow (Build and Test) failed during execution.

Error Details:

FAILURE: Build failed with an exception.

* Where:
Build file '/home/runner/work/beam-keyed-stream-sorting/beam-keyed-stream-sorting/build.gradle' line: 12

* What went wrong:
An exception occurred applying plugin request [id: 'com.diffplug.spotless', version: '8.7.0']
> Failed to apply plugin 'com.diffplug.spotless'.
   > Spotless requires Gradle 8.1 or newer, this was 7.6.6

Recommendation: Upgrade Gradle wrapper to 8.1+ before upgrading Spotless to 8.x, or pin Spotless to a version compatible with Gradle 7.6.6.

Leaving PR open and blocking merge until build failure is resolved.