Uses subclasses instead of descendants to support Rails < 7.1

We do override `subclasses` in railties to remove the abstract railties[1],
so in versions of rails that `descendants` is implemented in terms of `subclasses`
we get the same behavior, but in earlier versions of rails `descendants`
includes abstract railties which we don't want.

Fixes #737.

[1]: https://github.com/rails/rails/blob/b164bb832e8e11e7fb10f3c47d93ba29a98f2a94/railties/lib/rails/railtie.rb#L147-L149

Author: rafaelfranca

lib/spring/application.rb

@@ -129,8 +129,9 @@ def preload
 
       if defined?(Rails) && Rails.application
         watcher.add Rails.application.paths["config/initializers"]
-        Rails::Engine.descendants.each do |engine|
-          if engine.root.to_s.start_with?(Rails.root.to_s)
+        rails_root = Rails.root.to_s
+        Rails::Engine.subclasses.each do |engine|
+          if engine.root.to_s.start_with?(rails_root)
             watcher.add engine.paths["config/initializers"].expanded
           end
         end