Raise warnings for deprecated python cpp extension GetDebugString().

Pure python and upb do not support it and filtered out the test. This API does
not exists in any other language(except protobuf c++).
GetDebugString() for cpp extension will be removed in Jan 2025

PiperOrigin-RevId: 662640110

python/google/protobuf/pyext/descriptor.cc

@@ -1508,6 +1508,11 @@ static int SetHasOptions(PyFileDescriptor *self, PyObject *value,
 }
 
 static PyObject* GetDebugString(PyFileDescriptor* self) {
+  PyErr_Warn(nullptr,
+             "GetDebugString() API is deprecated. This API only "
+             "exists in protobuf c++ and does not exists in pure python, upb "
+             "or any other languages. GetDebugString() for python cpp "
+             "extension will be removed in Jan 2025");
   return PyString_FromCppString(_GetDescriptor(self)->DebugString());
 }