test: Add Rails 8.0 workflow

Appraisals

@@ -24,3 +24,8 @@ appraise 'rails72-activeadmin' do
   gem 'activeadmin'
   gem 'rails', '~> 7.2.0'
 end
+
+appraise 'rails80-activeadmin' do
+  gem 'activeadmin'
+  gem 'rails', '~> 8.0.0'
+end