🐛 Fix undefined method exists? error (#1524)

Fixes an error where `trellis up` or `vagrant up` would fail in machines
running Ruby on version 3.2 or higher due to the usage of a removed
method.

lib/trellis/vagrant.rb

@@ -105,7 +105,7 @@ def update_ssh_config(main_hostname)
   config_file = File.expand_path('~/.ssh/config')
   vagrant_ssh_config = `vagrant ssh-config --host #{main_hostname}`.chomp
 
-  if File.exists?(config_file)
+  if File.exist?(config_file)
     FileUtils.cp(config_file, "#{config_file}.trellis_backup")
     ssh_config = File.read(config_file)