Fix ruby3.2 deprecation exists? is now exist?

[augustosamame]

On Ruby versions 3.2 and greater, exists? is deprecated and has been replace with exist? Without this fix, the rails g two_factor_authentication MODEL generator will fail with:

undefined method `exists?' for File:Class (NoMethodError)

inject_into_file(path, "two_factor_authenticatable, :", :after => "devise :") if File.exists?(path)