boot menu

Grub2 has been the default boot loader for Ubuntu for more then 10 years now. Its configuration file is nowadays /boot/grub/grub.cfg, but you MUST NOT edit /boot/grub/grub.cfg in order to modify the Grub boot menu.
Instead, you need to add your own entries to the file /etc/grub.d/40_custom. Some general preferences are also set in the the file /etc/default/grub and any file under /etc/default/grub.d/.