Remove incorrect version constraint

Because Rails 7.1 is released, >= 7.0.0.alpha would install 7.1, and a
nokogiri version that doesn't work on Ruby 2.7. This caused 2.7 / 7.0 CI
to fail.

Also use >= 7.1.0.alpha in edge case, there is no alpha right now.

test/support/acceptance_test.rb

@@ -12,9 +12,7 @@ class AcceptanceTest < ActiveSupport::TestCase
 
       def rails_version
         if ENV['RAILS_VERSION'] == "edge"
-          "7.1.0.alpha"
-        elsif ENV['RAILS_VERSION'] == "7.0"
-          ">= 7.0.0.alpha"
+          ">= 7.1.0.alpha"
         else
           "~> #{ENV['RAILS_VERSION'] || "6.1"}.0"
         end