You could enter it in /home/nextcloud1/.local/share/containers/storage/volumes/nextcloud-app-data/_data/config/config.php but you’d need to check the owner.
To avoid owner issues, you could edit it by entering the app environment…
runagent -m nextcloud1
and edit config.php using nano:
podman unshare nano $(podman volume inspect nextcloud-app-data --format={{.Mountpoint}})/config/config.php
Another way instead of editing config.php is to use occ:
occ config:system:set trashbin_retention_obligation --value="auto, 60"
The setting will survive reboot, update and backup/restore.