TYPO3v11: Class TYPO3\CMS\Extbase\Object\ObjectManager is deprecated

When upgrading a TYPO3 from v10 to v11 I got the following deprecation notice:

TYPO3 Deprecation Notice: Class TYPO3CMSExtbaseObjectObjectManager is deprecated and will be removed in TYPO3 12.0 in /var/www/typo3/public/typo3/sysext/extbase/Classes/Object/ObjectManager.php line 104

The error stemmed from sysext/fluid - it could not load ViewHelpers via the symfony dependency container.

The solution was to define the namespace prefix in our extension's Configuration/Services.yaml:

Vendor\Extname\:
  resource: '../Classes/*'