Thank you!
Indeed on ubuntu 18.04 the previous command generated key.pem with first line -----BEGIN RSA PRIVATE KEY-----.
On ubuntu 20.04, the previous command generated key.pem with first line -----BEGIN OPENSSH PRIVATE KEY-----
Adding -m pem forces to generate key.pem with first line -----BEGIN RSA PRIVATE KEY-----.
Thank you! Indeed on ubuntu 18.04 the previous command generated key.pem with first line
-----BEGIN RSA PRIVATE KEY-----
. On ubuntu 20.04, the previous command generated key.pem with first line-----BEGIN OPENSSH PRIVATE KEY-----
Adding-m pem
forces to generate key.pem with first line-----BEGIN RSA PRIVATE KEY-----
.