Move `io.spine.tools.gradle.task.ProtobufTaskName` under the `protobuf` package

[alexander-yevsyukov]

Currently the class is misplaced. It comes under the package for general task extensions, while we have a dedicated package for things related to Protobuf.

The type is currently used in a couple of subprojects. So proper deprecation cycle will be necessary. Consider having temporarily open or sealed task in the protobuf package which the subject task extends. Then, when the deprecated task is deleted the one in the protobuf package would loose the modifier allowing the inheritance.