Conditionally overload direct_descendants

Rails 7.1 removed the deprecated method `direct_descendants` in favor
of `subclasses`

Subclasses implementation should be added in a different PR because it is not
trivial to implement

Ref #179

Ref:
- rails/rails@8f8aa85 rails/rails#39505
- rails/rails@1aa2463 rails/rails#46144

lib/chrono_model/time_machine.rb

@@ -21,9 +21,11 @@ module TimeMachine
       ChronoModel.history_models[table_name] = history
 
       class << self
-        alias_method :direct_descendants_with_history, :direct_descendants
-        def direct_descendants
-          direct_descendants_with_history.reject(&:history?)
+        if method_defined?(:direct_descendants)
+          alias_method :direct_descendants_with_history, :direct_descendants
+          def direct_descendants
+            direct_descendants_with_history.reject(&:history?)
+          end
         end
 
         alias_method :descendants_with_history, :descendants

spec/chrono_model/time_machine/sti_spec.rb

@@ -35,6 +35,22 @@ class ::Publication < Element
     it { is_expected.to include(Publication) }
   end
 
+  if ActiveRecord::Base.respond_to?(:direct_descendants)
+    describe '.direct_descendants' do
+      subject { Element.direct_descendants }
+
+      it { is_expected.to_not include(Element::History) }
+      it { is_expected.to     include(Publication) }
+    end
+
+    describe '.direct_descendants_with_history' do
+      subject { Element.direct_descendants_with_history }
+
+      it { is_expected.to include(Element::History) }
+      it { is_expected.to include(Publication) }
+    end
+  end
+
   describe 'timeline' do
     let(:publication) do
       pub = ts_eval { Publication.create! title: 'wrong title' }