account for the API differences in both rail6 and rails7

lib/rom/rails/active_record/configuration.rb

@@ -37,23 +37,19 @@ def initialize(env: ::Rails.env, root: ::Rails.root, configurations: ::ActiveRec
         #
         # @api private
         def call
-          specs = { default: build(default_configuration.symbolize_keys) }
+          specs = {}
 
-          if rails6?
-            configurations.configs_for(env_name: env).each do |config|
-              specs[config.spec_name.to_sym] = build(config.config.symbolize_keys)
-            end
+          if configurations.respond_to?(:default_hash)
+            specs[:default] = build(configurations.default_hash)
+          else
+            specs[:default] = build(configurations.configs_for(env_name: env).first.configuration_hash)
           end
 
-          specs
-        end
-
-        def default_configuration
-          if rails6?
-            configurations.default_hash(env)
-          else
-            configurations.fetch(env)
+          configurations.configs_for(env_name: env).each do |config|
+            specs[config.name.to_sym] = build(config.configuration_hash)
           end
+
+          specs
         end
 
         # Builds a configuration hash from a flat database config hash.
@@ -80,8 +76,12 @@ def build(config)
 
         private
 
+        def rails7?
+          ::ActiveRecord::VERSION::MAJOR == 7
+        end
+
         def rails6?
-          ::ActiveRecord::VERSION::MAJOR >= 6
+          ::ActiveRecord::VERSION::MAJOR == 6
         end
       end
     end