Enable frozen string for lib/gitlab/auth

Partially addresses:

https://gitlab.com/gitlab-org/gitlab/issues/27703

spec/lib/gitlab/auth/ldap/auth_hash_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Auth::LDAP::AuthHash do
@@ -91,7 +93,7 @@ describe Gitlab::Auth::LDAP::AuthHash do
       let(:given_uid) { 'uid=John Smith,ou=People,dc=example,dc=com' }
 
       before do
-        raw_info[:uid] = ['JOHN']
+        raw_info[:uid] = [+'JOHN']
       end
 
       it 'enabled the username attribute is lower cased' do

spec/lib/gitlab/auth/ldap/person_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Auth::LDAP::Person do
@@ -135,7 +137,7 @@ describe Gitlab::Auth::LDAP::Person do
       let(:username_attribute) { 'uid' }
 
       before do
-        entry[username_attribute] = 'JOHN'
+        entry[username_attribute] = +'JOHN'
         @person = described_class.new(entry, 'ldapmain')
       end