Update dependency checkstyle to v10.21.0 (#584)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

build.gradle

@@ -40,7 +40,7 @@ java {
 }
 
 checkstyle {
-  toolVersion = '10.20.2'
+  toolVersion = '10.21.0'
   configFile = new File(rootDir, "checkstyle.xml")
   maxWarnings = 0
 }