guests/centos: use GUEST_DETECTION_NAME='centos' to ID CentOS

This migrates the CentOS guest plugin to identify machines by
what is in os-release rather than based on if the /etc/centos-release
file exists.

It just so happens that the Rocky Linux plugin is set up to inherit
directly from :redhat (not :centos) and thus the CentOS flavor is
attempted before Rocky in the depth first search. For some reason
the rockylinux/8 vagrant box still has the /etc/centos-release file
in it even though it's only supposed to inherit from RHEL.

The almalinux/9 box does not have /etc/centos-release.

plugins/guests/centos/guest.rb

@@ -1,9 +1,11 @@
+require "vagrant"
+require_relative '../linux/guest'
+
 module VagrantPlugins
   module GuestCentos
-    class Guest < Vagrant.plugin("2", :guest)
+    class Guest < VagrantPlugins::GuestLinux::Guest
-      def detect?(machine)
+      # Name used for guest detection
-        machine.communicate.test("cat /etc/centos-release")
+      GUEST_DETECTION_NAME = "centos".freeze
-      end
     end
   end
 end