Force grub to boot default automatically:
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If grub refuses to take my changes with update-grub, I can manually edit the /boot/grub/grub.cfg file (I will lose changes evry time I update the kernel),
And change timeout to 0 and timeout style to hidden.

Date: Summer 2021


Update (Feb 6th 2024):
*As long that you have no other os installed (or Memtest86+), you can try this:

1. run: "sudo nano /etc/default/grub"
Set these 4 lines:
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0
GRUB_RECORDFAIL_TIMEOUT=0

2. Run: "sudo update-grub"

3. Reboot!
You should have no grub menu now.

sources:
https://askubuntu.com/questions/327311/how-to-auto-boot-to-ubuntu-instead-of-showing-text-mode-grub-menu-on-power-failu
https://superuser.com/questions/971704/changing-grub-timeout-value-doesn-t-seem-to-work