Bump com.diffplug.spotless:spotless-plugin-gradle from 6.20.0 to 6.21.0

Bumps [com.diffplug.spotless:spotless-plugin-gradle](https://github.com/diffplug/spotless) from 6.20.0 to 6.21.0.
- [Changelog](https://github.com/diffplug/spotless/blob/main/CHANGES.md)
- [Commits](diffplug/spotless@gradle/6.20.0...gradle/6.21.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

gradle/libs.versions.toml

@@ -17,7 +17,7 @@ mail = { module = "javax.mail:mail", version = "1.4.7"}
 nullaway = { module = "com.uber.nullaway:nullaway", version = "0.10.12" }
 protobuf-java = { module = "com.google.protobuf:protobuf-java", version.ref = "protobuf" }
 protobuf-java-util = { module = "com.google.protobuf:protobuf-java-util", version.ref = "protobuf" }
-spotless = { module = "com.diffplug.spotless:spotless-plugin-gradle", version = "6.20.0" }
+spotless = { module = "com.diffplug.spotless:spotless-plugin-gradle", version = "6.21.0" }
 
 [plugins]
 errorprone-plugin = { id = "net.ltgt.errorprone", version = "3.1.0" }