[Build] Update Gradle and gdx-jnigen

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,5 @@
-#Sat Dec 14 22:33:02 MSK 2019
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-bin.zip
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.8.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists