These two interfaces' methods have tentative return types in PHP 8.1,
which causes code without the type hints to raise warnings. Where the
type hint is "mixed", we need to use the special declaration
[\ReturnTypeWillChange] in a comment to suppress the warning as long as
we still support PHP < 8.0, which doesn't have a "mixed" type hint.
Bug: T289879
Change-Id: I1a126e602e92b8d13c7795eb6d790effd5ddc986