fix: Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-DEVISETWOFACTOR-8023206

Gemfile

@@ -31,7 +31,7 @@ gem 'browser'
 gem 'charlock_holmes', '~> 0.7.7'
 gem 'chewy', '~> 7.2'
 gem 'devise', '~> 4.8'
-gem 'devise-two-factor', '~> 4.0'
+gem 'devise-two-factor', '~> 6.0', '>= 6.0.0'
 
 group :pam_authentication, optional: true do
   gem 'devise_pam_authenticatable2', '~> 9.2'