protobuf_generate_cpp is deprecated

Used in https://github.com/llvm/llvm-project/blob/main/clang/tools/clang-fuzzer/CMakeLists.txt

```
protobuf_generate_cpp
Deprecated since version 4.1: Use [protobuf_generate()](https://cmake.org/cmake/help/latest/module/FindProtobuf.html#command:protobuf_generate).
```

https://cmake.org/cmake/help/latest/module/FindProtobuf.html
