Upgrade Gradle wrapper.

In turn implicitly upgrading the integrated plugin from Checkstyle version 8.12 to 8.27.

Author: Quintin Willison

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.2.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.6.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists