Find all usages of:

• public function getGroups()
• public function getGroupMemberships()
• public function getEffectiveGroups()
• public function getAutomaticGroups()
• public function getFormerGroups()
• public function addGroup()
• public function removeGroup()
• public function addAutopromoteOnceGroups())

and need to be replaced with corresponding UserGroupManager methods.
You can obtain UserGroupManager via MediaWikiServices::getInstance()->getUserGroupManager(), but it's preferred to inject it if possible.