Use minimum and maximum in enumerables

While the `minimum` and `maximum` methods have been available for a long
time for ActiveRecord relations, Rails 7.0 has added these methods for
enumerables as well.

This means that the `start_date` and `end_date` methods in the
ShiftsHelper can use `minimum` and `maximum` no matter whether they
receive an ActiveRecord relation or an array of polls (I think the
latter never happens, though, but I'm not 100% sure).

app/helpers/shifts_helper.rb

@@ -24,12 +24,12 @@ module ShiftsHelper
   end
 
   def start_date(polls)
-    start_date = polls.map(&:starts_at).min.to_date
+    start_date = polls.minimum(:starts_at).to_date
     [start_date, Date.current].max
   end
 
   def end_date(polls)
-    polls.map(&:ends_at).max.to_date
+    polls.maximum(:ends_at).to_date
   end
 
   def officer_select_options(officers)