[ticket/11888] Use \phpbb\search\fulltext_native as search backend config

If \ is missing, the default search backend shown in ACP is wrong.

PHPBB3-11888
[ticket/11888] Update default search backend in namespaces migration

PHPBB3-11888
Merge pull request #1753 from dhruvgoel92/ticket/11888

[ticket/11888] Use \phpbb\search\fulltext_native as search backend config