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* Remove outdated option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/lib/plugin/filter_manager_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Plugin::FilterManager do
+RSpec.describe Plugin::FilterManager do
   let(:instance) { Plugin::FilterManager.new }
 
   it "calls registered filters correctly" do

spec/lib/plugin/instance_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Plugin::Instance do
+RSpec.describe Plugin::Instance do
 
   after do
     DiscoursePluginRegistry.reset!

spec/lib/plugin/metadata_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Plugin::Metadata do
+RSpec.describe Plugin::Metadata do
   context "parse" do
     it "correctly parses plugin info" do
       metadata = Plugin::Metadata.parse <<TEXT