[7.x][ML] Upgrade to Gradle 6.8.3 (#1769)

Soon we will upgrade to version 7, and being on the
latest in the 6.x series reduces the risk of missing
a deprecation warning.

Backport of #1768

Author: droberts195

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.5.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.8.3-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionSha256Sum=143a28f54f1ae93ef4f72d862dbc3c438050d81bb45b4601eb7076e998362920
+distributionSha256Sum=9af5c8e7e2cd1a3b0f694a4ac262b9f38c75262e74a9e8b5101af302a6beadd7