By default, Ubuntu 20.04, or better the used OpenSSL version, disables all ciphers below TLSv1.2 and you need to re-enable it in order to use older ciphers. It is quite simple, add following text at the top of /etc/ssl/openssl.cnf:

openssl_conf = default_conf

[ default_conf ]

ssl_conf = ssl_sect

[ssl_sect]

system_default = system_default_sect

[system_default_sect]
MinProtocol = TLSv1.2
CipherString = DEFAULT:@SECLEVEL=1

Now reboot the machine and you should be done.

Warning: this can harm your machine, not physically but mentally. make sure you know how to restore the openssl.cnf if you have to.