Keep the constantize behavior consistent for versions prior to Rails 7

Use `AS::Dependencies` as before if we still can, otherwise use the new
direct `constantize` call for Rails 7+.

Leave a TODO to help remind us this can be removed once we drop support
to Rails versions prior to 7 in the future.

lib/devise.rb

@@ -313,11 +313,17 @@ def initialize(name)
     end
 
     def get
-      @name.constantize
+      # TODO: Remove AS::Dependencies usage when dropping support to Rails < 7.
+      if ActiveSupport::Dependencies.respond_to?(:constantize)
+        ActiveSupport::Dependencies.constantize(@name)
+      else
+        @name.constantize
+      end
     end
   end
 
   def self.ref(arg)
+    # TODO: Remove AS::Dependencies usage when dropping support to Rails < 7.
     if ActiveSupport::Dependencies.respond_to?(:reference)
       ActiveSupport::Dependencies.reference(arg)
     end