Replacement of check for base_excel

[garbast]

I would like to suggest to replace the check in

powermail/Classes/Controller/AbstractController.php

Line 113 in b23b0cb

$this->isPhpSpreadsheetInstalled = ExtensionManagementUtility::isLoaded('base_excel');

with an check for class_exists(\PhpOffice\PhpSpreadsheet\IOFactory)

By the changed check an installation which already uses phpoffice/phpspreadsheet for other tasks does not need to install base_excel to be able to export forms as xls. And TER users that rely on base_excel are fine too, because the factory is delivered by it.