Add RSpec 4 compatibility (#17652)

* Remove outdated option

04078317ba

* Use the non-globally exposed RSpec syntax

https://github.com/rspec/rspec-core/pull/2803

* Use the non-globally exposed RSpec syntax, cont

https://github.com/rspec/rspec-core/pull/2803

* Comply to strict predicate matchers

See:
 - https://github.com/rspec/rspec-expectations/pull/1195
 - https://github.com/rspec/rspec-expectations/pull/1196
 - https://github.com/rspec/rspec-expectations/pull/1277

spec/models/user_field_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe UserField do
+RSpec.describe UserField do
   describe "doesn't validate presence of name if field type is 'confirm'" do
     subject { described_class.new(field_type: 'confirm') }
     it { is_expected.not_to validate_presence_of :name }