Revert introduction of forwardable

Mongoid 7.1.0 introduced a regression (see
https://jira.mongodb.org/browse/MONGOID-4849) by switching to the
builtin ruby forwardable method of delegation rather than relying on the
ActiveSupport-provided delegation used before.

mongoid-locker was made compatible with the regressed version of mongoid
in mongodb/mongoid#4739

Mongoid then fixed this regression in 7.1.1+ with
mongodb/mongoid@d078acd,
which goes back to the active support way of delegating.

The `forwardable` inclusion in mongoid-locker causes incompatibilities
when you then include another gem that expects to be able to set up a
delegator within a mongoid document class. For example the
`mongoid-history` gem expects to be able to delegate using the
active-support style here:
https://github.com/mongoid/mongoid-history/blob/c8c4de1235c01252dfbdb9fe6c0abfc078ec1443/lib/mongoid/history/trackable.rb#L24-L25

This PR reverts the change to introduce `forwardable`.

See
mongoid/mongoid-history#238 (comment)
for an example of how this interacts poorly with mongoid-history.

Author: scpike

lib/mongoid/locker.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'forwardable'
 require 'securerandom'
 
 module Mongoid
@@ -104,8 +103,6 @@ def reset!
 
       # @api private
       def included(klass)
-        klass.extend(Forwardable) unless klass.ancestors.include?(Forwardable)
-
         klass.extend ClassMethods
         klass.singleton_class.instance_eval { attr_accessor(*MODULE_METHODS) }
 
@@ -117,7 +114,7 @@ def included(klass)
         klass.backoff_algorithm = backoff_algorithm
         klass.locking_name_generator = locking_name_generator
 
-        klass.def_delegators(klass, *MODULE_METHODS)
+        klass.delegate(*MODULE_METHODS, to: :class)
         klass.singleton_class.delegate(*(methods(false) - MODULE_METHODS.flat_map { |method| [method, "#{method}=".to_sym] } - %i[included reset! configure]), to: self)
       end
     end