Add shared_context for Rails 8.1

The development of Rails 8.1 has started in rails/rails:
rails/rails@592ea10

Author: koic

spec/support/shared_contexts.rb

@@ -39,3 +39,7 @@
 RSpec.shared_context 'with Rails 8.0', :rails80 do
   let(:rails_version) { 8.0 }
 end
+
+RSpec.shared_context 'with Rails 8.1', :rails81 do
+  let(:rails_version) { 8.1 }
+end