examples/agdk-mainloop: Use Gradle 8.4 (compatible with Java 21)

Author: rib

examples/agdk-mainloop/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 #Mon May 02 15:39:12 BST 2022
 distributionBase=GRADLE_USER_HOME
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.0-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.4-bin.zip
 distributionPath=wrapper/dists
 zipStorePath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME