Solve deprecations in t3o_membership

In order to update typo3.org to v9 as smoothly as possible, all extensions it requires should support v9 properly.

TYPO3Scanner result for t3o_membership:

Found 3 matches in 0.17s when checking for changes and deprecations in TYPO3 9

strong	weak	BREAKING	DEPRECATION
100.0% (3)	0.0% (0)	66.7% (2)	33.3% (1)

Classes/Task/ImportMembersTask.php
Access to global array "TYPO3_DB" (strong)
245 return $GLOBALS['TYPO3_DB'];
Breaking: #80929 - TYPO3_DB moved to extension
https://docs.typo3.org/typo3cms/extensions/core/Changelog/9.0/Breaking-80929-TYPO3_DBMovedToExtension.html

Use of static class method call "TYPO3\CMS\Core\Utility\GeneralUtility::getUserObj()" (strong)
230 $hookObject = GeneralUtility::getUserObj($classData);
Deprecation: #80993 - GeneralUtility::getUserObj
https://docs.typo3.org/typo3cms/extensions/core/Changelog/9.0/Deprecation-80993-GeneralUtilitygetUserObj.html

Classes/Domain/Repository/MemberRepository.php
Access to global array "TYPO3_DB" (strong)
68 return $GLOBALS['TYPO3_DB'];
Breaking: #80929 - TYPO3_DB moved to extension
https://docs.typo3.org/typo3cms/extensions/core/Changelog/9.0/Breaking-80929-TYPO3_DBMovedToExtension.html

These deprecations should either be solved in the update branch update-v9 directly, or have a Merge Request pointed to that branch in the t3o_membership repository.