Add gradle wrapper to `java/`

@cblichmann
Please run `gradle wrapper` on your local developer machine to generate a `gradlew` file in `java/` to lock the Gradle version in this repository.

I first tried Gradle 6.8.3 (as stated in README.md) but it actually requires Gradle 8.0+:

```
FAILURE: Build failed with an exception.

* Where:
Build file '/home/mochaa/ghq/github.com/google/bindiff/java/ui/build.gradle' line: 27

* What went wrong:
A problem occurred evaluating project ':ui'.
> Failed to apply plugin class 'com.github.jengelman.gradle.plugins.shadow.ShadowBasePlugin'.
> This version of Shadow supports Gradle 8.0+ only. Please upgrade.

* 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
```