[sdk-platform-java] Migrate the generated Gradle files to be compatible with Java 22

The generated Gradle files as part of the self service(Bazel) flow includes configurations that are not compatible with Java 22.

1. The gradle-wrapper.properties references 7.6.4 which does not support Java 22. The earliest Gradle version that supports Java 22 is 8.8 per [compatibility table](https://docs.gradle.org/current/userguide/compatibility.html).
2. There are a few templates reference [classifier](https://github.com/googleapis/sdk-platform-java/blob/main/rules_java_gapic/resources/gradle/grpc.gradle.tmpl#L20), which has been deprecated since 5.1 and removed since 8.0.

We need to migrate these files to be compatible with Java 22, but also stay compatible with Java 8 and Gradle 7.x.