Update dependency checkstyle to v10.20.2 (#826)

Co-authored-by: grafanarenovatebot[bot] <146220760+grafanarenovatebot[bot]@users.noreply.github.com>

gradle/checkstyle.gradle

@@ -3,7 +3,7 @@ apply plugin: "checkstyle"
 checkstyle {
   configFile = rootProject.file("buildscripts/checkstyle.xml")
   // this version should match the version of google_checks.xml used as basis for above configuration
-  toolVersion = "10.20.1"
+  toolVersion = "10.20.2"
   maxWarnings = 0
   checkstyleTest.enabled = false
 }