grub: T6165: increase service TimeoutSec from 5 -> 60

The PCEngines APU2 systems with mSATA disks tend to be very slow. This results
in a service startup error:

$ systemctl status vyos-grub-update
× vyos-grub-update.service - Update GRUB loader configuration structure

  Loaded: loaded (/lib/systemd/system/vyos-grub-update.service; enabled; preset: enabled)
  Active: failed (Result: timeout) since Sun 2024-03-24 08:48:10 UTC; 14min ago
Main PID: 779 (code=killed, signal=TERM)
     CPU: 869ms

Mar 24 08:48:05 LR4.wue3 systemd[1]: Starting vyos-grub-update.service - Update GRUB loader configuration structure...
Mar 24 08:48:10 LR4.wue3 systemd[1]: vyos-grub-update.service: start operation timed out. Terminating.
Mar 24 08:48:10 LR4.wue3 systemd[1]: vyos-grub-update.service: Main process exited, code=killed, status=15/TERM
Mar 24 08:48:10 LR4.wue3 systemd[1]: vyos-grub-update.service: Failed with result 'timeout'.
Mar 24 08:48:10 LR4.wue3 systemd[1]: Failed to start vyos-grub-update.service - Update GRUB loader configuration structure.

Measunring on an APU2 system after boot and memory is "hot", it still needs
almost 17 seconds to complete the job

cpo@LR4.wue3:~$ time sudo /usr/libexec/vyos/system/grub_update.py
real 0m16.803s
user 0m0.018s
sys 0m0.028s

(cherry picked from commit 5a12645cb25fb23f2195db1e2e977a69d0788d01)