Introduce Adapter::Base

Breaking change:
- Adapters now inherit Adapter::Base
- 'Adapter' is now a module, no longer a class
Why?

- using a class as a namespace that you also inherit from is complicated and circular at time i.e.
  buggy (see https://github.com/rails-api/active_model_serializers/pull/1177)
- The class methods on Adapter aren't necessarily related to the instance methods, they're more
    Adapter functions
- named `Base` because it's a Rails-ism
- It helps to isolate and highlight what the Adapter interface actually is

test/adapter/json_api/linked_test.rb

@@ -1,7 +1,7 @@
 require 'test_helper'
 module ActiveModel
   class Serializer
-    class Adapter
+    module Adapter
       class JsonApi
         class LinkedTest < Minitest::Test
           def setup