Changing approval rule target branch should not affect existing merge requests

The discussion !24344 (comment 285839848) should be addressed.

As explained in the docs: “Any merge requests that were created before changing the rules will not be changed. They will keep the original approval rules, unless manually overridden.” This is correct if I change the name, approvers, and no. of required approvals in the project settings. But if I change the approval rule's target branch in the project settings, this affects existing merge requests, which it shouldn't per the docs.

[…] changing anything about the project-level approval rules should not affect existing merge requests. As it is right now, changing the target branch of a project-level approval rule affects existing merge requests, which goes against the documented behavior.