To find the version of OpenSSH your PAN-OS device is using see Palo Alto Networks Open-Source Software (OSS) Licenses and select the PAN-OS version your device is on to the left.
https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClgBCAS&refURL=http%3A%2F%2Fknowledgebase.paloaltonetworks.com%2FKCSArticleDetail