disabling the self-update API point

[OniriCorpe]

Problem

• Once, I tried to update my AGH using the built-in self-updater, and that completely broke my AGH and I haven't figured out how to fix

Solution

• So, better disabling it, it’s kinda useless with YNH tho

If someone try to self-update AdGuard Home, nginx returns 403 Forbidden

PR Status

• [x] Code finished and ready to be reviewed/tested
• [ ] The fix/enhancement were manually tested (if applicable)