Lock Rails to v6 (#22)

We haven't tested this with Rails 7 and Rails 7 has some auto reconnect
and retry features for read queries too, so part of the feature set is also covered

rails-pg-adapter.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |spec|
   end
   spec.require_paths = ["lib"]
 
-  spec.add_dependency("rails", "~> 6")
+  spec.add_dependency("rails", "~> 6", "<7")
 
   spec.metadata = { "rubygems_mfa_required" => "true" } if spec.respond_to?(:metadata=)
 end