TLS 1.2 disable in proxmox backup server version 4

i don't think it's possible to configure tls1.3 only is there a specific reason why you want to do this?
 
You could try updating the /etc/ssl/openssl.conf file.
Add the ssl_conf line at the right section, [openssl_init] already exists.

Code:
[openssl_init]
ssl_conf = ssl_sect

[ssl_sect]
system_default = system_default_sect

[system_default_sect]
MinProtocol = TLSv1.3