Stop supporting `require ‘rspec-expectations’`.

Use `require ‘rspec/expectations’` instead.

We don't have `rspec-mocks.rb` or `rspec-core.rb` files for those libs.  We don't need it for this lib, and its presence has made us do some one-off work arounds:

https://github.com/rspec/rspec-support/blob/17bd12c37c4ae608490afd2db75d7b04b3cc7171/spec/rspec/support/caller_filter_spec.rb#L27

https://github.com/rspec/rspec-expectations/blob/2ef8bc86e7523616cb3719004fe08a6bfe55f741/.rubocop.yml#L34