update the Gradle wrapper to v7.6.4 (jMonkeyEngine#2197)

Author: stephengold

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-7.6.3-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.6.4-bin.zip
 networkTimeout=10000
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists