Remove usage of deprecated protobuf method (protobuf::compiler::java::ClassName) in java_generator.cpp

[sophiechang]

What version of gRPC-Java are you using?

HEAD

What is your environment?

N/A

What did you expect to see?

Deprecated function not used

See protocolbuffers/protobuf@ca4fb2f for update

What did you see instead?

Deprecated function used here
https://github.com/grpc/grpc-java/blob/master/compiler/src/java_plugin/cpp/java_generator.cpp#L216C10-L216C45

Steps to reproduce the bug

Compile and treat deprecated-declarations as error