Open the Terminal (via Spotlight or through the Utilities folder) and enter the following command string:

sudo nano /etc/ssh/ssh_config

You’ll need to authenticate with the admin password.

Scroll all the way to the bottom of the ssh_config file and then add the following lines to the bottom of ssh_config:

HostkeyAlgorithms +ssh-rsa
PubkeyAcceptedAlgorithms +ssh-rsa

Hit Control+O to save, and Control+X to exit.

Try using SSH to connect to a server with RSA keys again, it should work as intended.