Services can be defined on the root level of .services.yml files

Since #2139583: Clarify error message when services.yml definitions are missing top-level keys you can define services without having to start with a 'services' key as the root level.

Parameters can still be defined under the 'parameters' key.

Before

services:
  access_check.permission:
    class: Drupal\user\Access\PermissionAccessCheck
    tags:
      - { name: access_check, applies_to: _permission }
  access_check.user.register:
    class: Drupal\user\Access\RegisterAccessCheck
    tags:
      - { name: access_check, applies_to: _access_user_register }

After

access_check.permission:
  class: Drupal\user\Access\PermissionAccessCheck
  tags:
    - { name: access_check, applies_to: _permission }
access_check.user.register:
  class: Drupal\user\Access\RegisterAccessCheck
  tags:
    - { name: access_check, applies_to: _access_user_register }