Revert "Update to Gradle 6.1"

This reverts commit 7f9715d9513280339065a329f52dbc09b0b2aad3.

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.6.2-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists