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* Remove outdated option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

plugins/discourse-local-dates/spec/lib/pretty_text_spec.rb

@@ -14,7 +14,7 @@ def generate_html(text, opts = {})
   output + "</span></p>"
 end
 
-describe PrettyText do
+RSpec.describe PrettyText do
   before do
     freeze_time
   end

plugins/discourse-local-dates/spec/models/post_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Post do
+RSpec.describe Post do
 
   before do
     Jobs.run_immediately!