[Task] Dubbo is compatible with old version specifications

At present, it is found that in the development process of Dubbo related functions, there are behaviors such as directly deleting methods, types, modifying package names, etc. that are incompatible with older versions. It is necessary to specify compatibility specifications to ensure that each partner who submits code follows this specification.

No file, test, or entry point is named. Start by locating the relevant Dubbo website documentation and clarify which compatibility behaviors and older versions the specification must cover; done means a documented compatibility specification that contributors can follow.

Written by the indexing model from the issue text.