Use the rails 7.1+ serialize interface

Type was changed from positional to a keyword argument 'type' in 7.1.
The positional argument is removed in rails 7.2.

We have a helper bridge module prepending support for either kwargs
or positional arguments passed to serialize which converts to positional
for rails 7.0 and kwargs for 7.1+.  This was added and automatically
loaded in the rails app via the engine in schema:
ManageIQ/manageiq-schema#756

Similar change as core: ManageIQ/manageiq#23253

app/models/manageiq/showback/input_measure.rb

@@ -2,7 +2,7 @@ module ManageIQ::Showback
   class InputMeasure < ApplicationRecord
     validates :description, :entity, :group, :fields, :presence => true
 
-    serialize :fields, Array
+    serialize :fields, :type => Array
 
     self.table_name = "showback_input_measures"