[#1621] Upgrade gradle spotless plugin to 6.18.0

Author: DavideD

build.gradle

@@ -3,7 +3,7 @@ import java.util.regex.Pattern
 plugins {
     id 'java-library'
     id 'maven-publish'
-    id 'com.diffplug.spotless' version '6.11.0'
+    id 'com.diffplug.spotless' version '6.18.0'
     id 'nu.studer.credentials' version '2.1'
     id 'org.asciidoctor.jvm.convert' version '3.3.2' apply false
     id 'io.github.gradle-nexus.publish-plugin' version '1.1.0'