rubocop noise: fixes FrozenStringLiteralComment

Adding the .ruby-verison file triggered previously un-run cops, specifically:

  This cop is designed to help upgrade to Ruby 3.0. It will add the
  comment `# frozen_string_literal: true` to the top of files to enable
  frozen string literals. Frozen string literals will be default in Ruby
  3.0. The comment will be added below a shebang and encoding comment. The
  frozen string literal comment is only valid in Ruby 2.3+.

More info on rubocop [Automatic-Corrections](https://github.com/bbatsov/rubocop/wiki/Automatic-Corrections)

app/models/answer.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class Answer < ApplicationRecord
   serialize :answer
 

app/models/application_record.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class ApplicationRecord < ActiveRecord::Base
   self.abstract_class = true
 

app/models/candidate.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class Candidate < ApplicationRecord
   belongs_to :quiz
   has_many :questions, -> { order("sort") }, through: :quiz

app/models/question.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class Question < ApplicationRecord
   serialize :input_options
 

app/models/quiz.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class Quiz < ApplicationRecord
   has_many :questions, -> { order(:sort) }
   has_many :candidates

app/models/user.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 class User < ApplicationRecord
   has_secure_password
   has_many :candidates, foreign_key: "recruiter_id"