missing require: the AR session store depends on the AP abstract store

This require makes the dependency even more clear.
In particular we are eager loading the session
store but that does not work if AR is used
outside Rails, this patch is preliminary work
in fixing #7160.

activerecord/lib/active_record/session_store.rb

+require 'action_dispatch/middleware/session/abstract_store'
+
 module ActiveRecord
   # = Active Record Session Store
   #