chore: Constrain activesupport < 7.2 for Grape 1.8.0 compatibility

Grape 1.8.0 is incompatible with activesupport >= 7.2 due to deprecation API
changes. Add conditional constraint to allow testing with Grape 1.8.0 while
maintaining compatibility with Grape 2.x and 3.x.

Author: numbata

Gemfile

@@ -25,6 +25,7 @@ group :development, :test do
     gem 'rack', '>= 3.0'
   else
     gem 'rack', '< 3.0'
+    gem 'activesupport', '< 7.2'
   end
 
   gem 'cgi'