Bump com.diffplug.spotless:spotless-plugin-gradle from 6.18.0 to 6.20.0

Bumps [com.diffplug.spotless:spotless-plugin-gradle](https://github.com/diffplug/spotless) from 6.18.0 to 6.20.0.
- [Changelog](https://github.com/diffplug/spotless/blob/main/CHANGES.md)
- [Commits](diffplug/spotless@gradle/6.18.0...gradle/6.20.0)

---
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]>

build.gradle

@@ -7,7 +7,7 @@ buildscript {
             // Code formatting; defines targets "spotlessApply" and "spotlessCheck".
             // https://github.com/diffplug/spotless/tags ; see tags starting "gradle/"
             // Only works on JDK 11+.
-            classpath 'com.diffplug.spotless:spotless-plugin-gradle:6.18.0'
+            classpath 'com.diffplug.spotless:spotless-plugin-gradle:6.20.0'
         }
     }
 }