`array_key_first` is only supported in PHP 7.3 and above.

Problem/Motivation

The module import fails when using PHP version less that 7.3 because the `array_key_first` is only available with 7.3 and above.

Proposed resolution

Replace the `array_key_first` function use with a custom polyfil.

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Error: Call to undefined function Drupal\excel_importer\Form\array_key_first() in Drupal\excel_importer\Form\ExcelImporterForm->getCorrectValue() (line 288 of modules/contrib/excel_importer/src/Form/ExcelImporterForm.php).