Upgrade gradlew to gradle 7.6.4

[EnricoMi]

Current build.gradle does not work with Gradle 6:

FAILURE: Build failed with an exception.

* Where:
Build file '.../dgraph4j/build.gradle' line: 46

* What went wrong:
A problem occurred evaluating root project 'dgraph4j'.
> Could not find method base() for arguments [build_v9fysx0wd2y7qeqfcjc5a1h4$_run_closure1@4c74018b] on root project 'dgraph4j' of type org.gradle.api.Project.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 467ms

So gradlew should reference a version that works with it.

Could also upgrade to Gradle 8.

[EnricoMi]

Github CI just works because it installs latest Gradle and uses that.