In #42843 and #42846, several config settings were added to control the
default serializer for `MessageEncryptor` and `MessageVerifier`, and to
provide a migration path from a default `Marshal` serializer to a
default `JSON` serializer:

* `config.active_support.default_message_encryptor_serializer`
  * Supports `:marshal`, `:hybrid`, or `:json`.
* `config.active_support.default_message_verifier_serializer`
  * Supports `:marshal`, `:hybrid`, or `:json`.
* `config.active_support.fallback_to_marshal_deserialization`
  * Affects `:hybrid` for both `MessageEncryptor` and `MessageVerifier`.
* `config.active_support.use_marshal_serialization`
  * Affects `:hybrid` for both `MessageEncryptor` and `MessageVerifier`.

This commit unifies those config settings into a single setting,
`config.active_support.message_serializer`, which supports `:marshal`,
`:json_allow_marshal`, and `:json` values.  So, for example,

  ```ruby
  config.active_support.default_message_encryptor_serializer = :hybrid
  config.active_support.default_message_verifier_serializer = :hybrid
  config.active_support.fallback_to_marshal_deserialization = true
  config.active_support.use_marshal_serialization = false
  ```

becomes

  ```ruby
  config.active_support.message_serializer = :json_allow_marshal
  ```

and

  ```ruby
  config.active_support.default_message_encryptor_serializer = :hybrid
  config.active_support.default_message_verifier_serializer = :hybrid
  config.active_support.fallback_to_marshal_deserialization = false
  config.active_support.use_marshal_serialization = false
  ```

becomes

  ```ruby
  config.active_support.message_serializer = :json
  ```

This commit also replaces `ActiveSupport::JsonWithMarshalFallback` with
`ActiveSupport::Messages::SerializerWithFallback`, which implements a
generic mechanism for serializer fallback.  The `:marshal` serializer
uses this mechanism too, so

  ```ruby
  config.active_support.default_message_encryptor_serializer = :hybrid
  config.active_support.default_message_verifier_serializer = :hybrid
  config.active_support.fallback_to_marshal_deserialization = false
  config.active_support.use_marshal_serialization = true
  ```

becomes

  ```ruby
  config.active_support.message_serializer = :marshal
  ```

Additionally, the logging behavior of `JsonWithMarshalFallback` has been
replaced with notifications which include the names of the intended and
actual serializers, as well as the serialized and deserialized message
data.  This provides a more targeted means of tracking serializer
fallback events.  It also allows the user to "silence" such events, if
desired, without an additional config setting.

All of these changes make it easier to add migration paths for new
serializers such as `ActiveSupport::MessagePack`.