How to switch back networking to /etc/network/interfaces on Ubuntu 20.04 Focal Fossa Linux - LinuxConfig.org

This article will explain how to switch back networking from NetPlan/CloudInit on Ubuntu 20.04 Focal Fossa Linux to yet now already obsolete networking managed via /etc/network/interfaces.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-switch-back-networking-to-etc-network-interfaces-on-ubuntu-20-04-focal-fossa-linux