addition of return type to various __sleep() methods needs BC

Problem/Motivation

#3425537: Fix remaining methods with #[\ReturnTypeWillChange] attribute added a return value to various implementations of __sleep().

This makes contrib modules crash on 11.x

We should add a deprecation notice on 10.x.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet