Migrate old APIs to new ones
On some places old APIs were used, which do not exist anymore, e.g. calls like Tx_Extbase_Utility_Cache::clearPageCache($pages); which should now be
\TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('\TYPO3\CMS\Extbase\Object\ObjectManager') ->get('\TYPO3\CMS\Extbase\Service\CacheService') ->clearPageCache($pages);
This can be migrated in one step to ease migration of extensions.