Upgrade to Gradle Wrapper 8.8

Not quite an entry, but the Gradlew wrapper should be updated to 8.8 so it can run on JDK 22 (it's 8.7 currently).

Also, Gradle projects should be configured with a Java Toolchain instead of `sourceCompatibility`.

```kotlin
javaToolchains {
	version = JavaVersion.VERSION_22
}
```