It should be in conf, the documentation states it here, https://experienceleague.adobe.com/en/docs/experience-manager-cloud-service/content/operations/maintenance?lang=en#purge_tasks
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It Quotes:
Must be done in git. Override the out-of-the-box Maintenance window configuration node under /libs by creating properties under the folder /conf/global/settings/granite/operations/maintenance/granite_weekly, granite_daily or granite_monthly. See the Maintenance Window table below for additional configuration details.
Enable the maintenance task by adding another node under the node above (name it granite_WorkflowPurgeTask) with the appropriate properties. Configure the OSGI properties, see Regular Purging of Workflow Instances.
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In AEMaaCS, the official documentation clearly states that workflow purge configuration must be created under the /conf directory, and this configuration must be committed to Git and deployed through Cloud Manager. In Cloud Service, you cannot modify /libs, as it is immutable and managed by Adobe.
Therefore, when the documentation says to “override the out-of-the-box Maintenance window configuration node under /libs,” it means you should create the corresponding configuration structure under /conf/global/settings/granite/operations/maintenance/ to override the default settings provided by Adobe.
These folders exist conceptually in /libs, but in AEMaaCS you override them by creating the same structure under /conf. Then, to enable workflow purging, you add a child node (for example, granite_WorkflowPurgeTask) under the selected maintenance window node and configure it with the appropriate properties. These properties define how the workflow instances are purged, such as how many days old they must be and which workflow statuses should be targeted.