Add and apply Style/MinMaxComparison rubocop rule

This rule was added in rubocop 1.42.0.

.rubocop.yml

@@ -646,6 +646,9 @@ Style/MapToHash:
 Style/MethodDefParentheses:
   Enabled: true
 
+Style/MinMaxComparison:
+  Enabled: true
+
 Style/MutableConstant:
   Enabled: true
 

app/helpers/shifts_helper.rb

@@ -9,7 +9,7 @@ module ShiftsHelper
     return [] if polls.blank?
 
     dates = polls.map(&:ends_at).map(&:to_date).sort.reduce([]) do |total, date|
-      initial_date = date < Date.current ? Date.current : date
+      initial_date = [date, Date.current].max
       total << (initial_date..date + Poll::RECOUNT_DURATION).to_a
     end
     date_options(dates.flatten.uniq, Poll::Shift.tasks[:recount_scrutiny], booth)
@@ -25,7 +25,7 @@ module ShiftsHelper
 
   def start_date(polls)
     start_date = polls.map(&:starts_at).min.to_date
-    start_date < Date.current ? Date.current : start_date
+    [start_date, Date.current].max
   end
 
   def end_date(polls)