Add and apply Style/ComparableBetween RuboCop rule

This rule was introduced in RuboCop 1.74 to prefer using between?
over chained comparison operators.

.rubocop.yml

@@ -725,6 +725,9 @@ Style/ClassVars:
 Style/CollectionMethods:
   Enabled: true
 
+Style/ComparableBetween:
+  Enabled: true
+
 Style/DigChain:
   Enabled: true
 

app/models/poll.rb

@@ -116,7 +116,7 @@ class Poll < ApplicationRecord
   end
 
   def current?(timestamp = Time.current)
-    starts_at <= timestamp && timestamp <= ends_at
+    timestamp.between?(starts_at, ends_at)
   end
 
   def expired?(timestamp = Time.current)