Bump com.diffplug.spotless from 8.1.0 to 8.2.1 in /lib/kotlin

Bumps com.diffplug.spotless from 8.1.0 to 8.2.1.

---
updated-dependencies:
- dependency-name: com.diffplug.spotless
  dependency-version: 8.2.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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