Is there a way to Restrict SSLVPN Connection from a Specific FortiClient Version?

This should be possible with the EMS, the free version might not be able to do this. Use the search engine for posture checks or compliance guides on FortiGate:

https://docs.fortinet.com/document/forticlient/6.0.0/compliance-guide/975855/fortigate-compliance-rules

There might be more. However, this does not look like you could exclude one specific version, more like zone out client older than X.

Best regards,

Markus