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)

test/mailers/previews/candidate_mailer_preview.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # Preview all emails at http://localhost:3000/rails/mailers/candidate_mailer
 class CandidateMailerPreview < ActionMailer::Preview
   def welcome

test/mailers/previews/recruiter_mailer_preview.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # Preview all emails at http://localhost:3000/rails/mailers/recruiter_mailer
 class RecruiterMailerPreview < ActionMailer::Preview
   def candidate_created

test/mailers/previews/reviewer_mailer_preview.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # Preview all emails at http://localhost:3000/rails/mailers/reviewer_mailer
 class ReviewerMailerPreview < ActionMailer::Preview
   def candidate_submission

test/mailers/previews/user_mailer_preview.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # Preview all emails at http://localhost:3000/rails/mailers/user_mailer
 class UserMailerPreview < ActionMailer::Preview
   def password_reset