Clear voter count when poll is reset (#21700)

When a poll is edited, we reset the poll and remove all previous
votes. However, prior to this commit, the voter count on the poll
was not reset. This leads to incorrect percentages being shown in
poll results.

Fixes #21696

app/models/poll.rb

@@ -85,6 +85,7 @@ class Poll < ApplicationRecord
   def reset_votes!
     self.cached_tallies = options.map { 0 }
     self.votes_count = 0
+    self.voters_count = 0
     votes.delete_all unless new_record?
   end