config: change global_namespace_import to true

https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/2.19/doc/rules/import/global_namespace_import.rst

Author: kenjis

src/CodeIgniter4.php

@@ -171,7 +171,7 @@ public function __construct()
  'fix_inline' => true,
  'replacements' => ['inheritDocs' => 'inheritDoc'],
  ],
- 'global_namespace_import' => false,
+ 'global_namespace_import' => true,
  'group_import' => false,
  'header_comment' => false, // false by default
  'heredoc_indentation' => ['indentation' => 'start_plus_one'],