update-grub does not run on kernel updates

[Bas van der Lei]

A fresh Ubuntu 18.04.5 installation (eg. from ubuntu-18.04.5-server-amd64.iso) on a EFI machine, will not run update-grub when a kernel is installed or removed. Resulting in an older kernel being booted or missing/broken entries in grub.cfg.

It seems the following pre/post hook scripts are missing:
    - `/etc/kernel/postinst.d/zz-update-grub`
    - `/etc/kernel/postrm.d/zz-update-grub`

This is due to a version mismatch of grub-efi-amd64 and grub2-common. In 2.04-1ubuntu44 of grub2 these scripts are installed as part of grub2-common package, while in 2.02-2ubuntu8.23 they are conffiles of the grub-efi-amd64 (or grub-pc, etc) package.

NOTE: A system that previously had an older version of grub-efi-amd64 or the grub-pc package installed will not encounter this issue since the scripts are still present as obsolete conffiles:

```
# dpkg --status grub-efi-amd64
<snip>
Conffiles:
 /etc/kernel/postrm.d/zz-update-grub 536d9d45e3e547638db3c5d58a925b6c obsolete
 /etc/kernel/postinst.d/zz-update-grub 536d9d45e3e547638db3c5d58a925b6c obsolete
```

---

```
$ lsb_release -rd
Description: Ubuntu 18.04.5 LTS
Release: 18.04

$ apt-cache policy grub-efi-amd64 grub2-common
grub-efi-amd64:
  Installed: 2.04-1ubuntu44
  Candidate: 2.04-1ubuntu44
  Version table:
 *** 2.04-1ubuntu44 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     2.02-2ubuntu8.17 500
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
     2.02-2ubuntu8 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages
grub2-common:
  Installed: 2.02-2ubuntu8.23
  Candidate: 2.02-2ubuntu8.23
  Version table:
 *** 2.02-2ubuntu8.23 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     2.02-2ubuntu8.17 500
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
     2.02-2ubuntu8 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages
```

---

Could also be the cause of #1922234 and #1927857