Add deprecation warnings for features to be removed/changed in RSpec 4
Created by: pirj
According to https://github.com/rspec/rspec-mocks/blob/4-0-dev/Changelog.md, only those two deprecation warnings are needed.
This is purposed to be released as version 3.99. Similar PRs for Expectations will follow. Sibling PRs:
Release strategy: https://github.com/rspec/rspec-core/pull/2880#issuecomment-797756681 See https://github.com/rspec/rspec/issues/61 RSpec 4 plan: https://github.com/rspec/rspec/issues/61 Changes https://github.com/rspec/rspec-mocks/blob/4-0-dev/Changelog.md#development