Fix the constantize method

Initially I've thought about adding a [managed string inflector](https://github.com/dry-rb/dry-inflector/blob/main/lib/dry/inflector.rb#L88) as a dependency, but there is no need for that just to use this single method.

Instead I've replaced the  with the  to avoid additional dependencies.
Resolves: #407

lib/sidekiq-scheduler/schedule.rb

@@ -148,9 +148,7 @@ def infer_queue(klass)
     end
 
     def try_to_constantize(klass)
-      klass.is_a?(String) ? klass.constantize : klass
-    rescue NameError
-      klass
+      SidekiqScheduler::Utils.try_to_constantize(klass)
     end
   end
 end

lib/sidekiq-scheduler/utils.rb

@@ -49,7 +49,7 @@ def self.symbolize_keys(object)
     #
     # @return [Class] the class corresponding to the klass param
     def self.try_to_constantize(klass)
-      klass.is_a?(String) ? klass.constantize : klass
+      klass.is_a?(String) ? Object.const_get(klass) : klass
     rescue NameError
       klass
     end