Merge pull request #313 from domaframework/spotless-3.20

Upgrade spotless-plugin-gradle to 3.20.0

Author: nakamura-to

build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'com.diffplug.gradle.spotless' version '3.16.0'
+    id 'com.diffplug.gradle.spotless' version '3.20.0'
     id 'java'
     id 'signing'
     id 'maven'