Drupal 9 readiness

$ vendor/bin/drupal-check -d modules/contrib/search_api_algolia/
3/3 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

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Line src/Plugin/search_api/backend/SearchApiAlgoliaBackend.php
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278 Call to deprecated method strlen() of class Drupal\Component\Utility\Unicode:
in drupal:8.6.0 and is removed from drupal:9.0.0. Use
mb_strlen() instead.
279 Call to deprecated method substr() of class Drupal\Component\Utility\Unicode:
in drupal:8.6.0 and is removed from drupal:9.0.0. Use
mb_substr() instead.
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[ERROR] Found 2 errors