Solve deprecations in randombanners

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

TYPO3Scanner result for randombanners:

Found 1 matches in 0.13s when checking for changes and deprecations in TYPO3 9

strong	weak	BREAKING
0.0% (0)	100.0% (1)	100.0% (1)

Classes/Controller/BannerController.php
Call to method "getContentObject()" (weak)
42 $this->view->assign('contentData', $this->configurationManager->getContentObject()->data);
Breaking: #80700 - Deprecated functionality removed
https://docs.typo3.org/typo3cms/extensions/core/Changelog/9.0/Breaking-80700-DeprecatedFunctionalityRemoved.html
Deprecation: #68748 - Deprecate AbstractContentObject::getContentObject()
https://docs.typo3.org/typo3cms/extensions/core/Changelog/8.0/Deprecation-68748-DeprecateAbstractContentObjectgetContentObject.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 randombanners repository.