arm64: not able to install linux-generic-hwe-18.04-edge/bionic-proposed

1. linux-generic-hwe-18.04-edge from bionic-updates is installed:
`linux-generic-hwe-18.04-edge/bionic-updates,bionic-security,now 4.18.0.16.65 arm64 [installed]`

```
$ dpkg -l |grep linux-generic
ii linux-generic-hwe-18.04-edge 4.18.0.16.65 arm64 Complete Generic Linux kernel and headers
```

2. enable -proposed archive and update:
```
$ sudo add-apt-repository "deb http://ports.ubuntu.com/ubuntu-ports/ $(lsb_release -sc)-proposed main restricted"
$ sudo apt update
```

3. shows it is upgradable:
```
$ apt search --names-only linux-generic
linux-generic-hwe-18.04-edge/bionic-proposed 5.0.0.8.66 arm64 [upgradable from: 4.18.0.16.65]
  Complete Generic Linux kernel and headers
```

4. try to install it, but:
```
$ sudo apt install linux-generic-hwe-18.04-edge
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 linux-generic-hwe-18.04-edge : Depends: linux-image-generic-hwe-18.04-edge (= 5.0.0.8.66) but 4.18.0.16.65 is to be installed
E: Unable to correct problems, you have held broken packages.
```

5. same happens trying to install `linux-image-generic-hwe-18.04-edge`:
```
$ sudo apt install linux-image-generic-hwe-18.04-edge
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 linux-image-generic-hwe-18.04-edge : Depends: linux-image-5.0.0-8-generic but it is not installable
E: Unable to correct problems, you have held broken packages.
```

6. also not able to install `linux-image-5.0.0-8-generic`:
```
$ sudo apt install linux-image-5.0.0-8-generic
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package linux-image-5.0.0-8-generic is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'linux-image-5.0.0-8-generic' has no installation candidate
```

more info:

~$ lsb_release -rd
Description: Ubuntu 18.04.2 LTS
Release: 18.04

~$ apt-cache policy linux-generic-hwe-18.04-edge
linux-generic-hwe-18.04-edge:
  Installed: 4.18.0.16.65
  Candidate: 5.0.0.8.66
  Version table:
     5.0.0.8.66 500
        500 http://ports.ubuntu.com/ubuntu-ports bionic-proposed/main arm64 Packages
 *** 4.18.0.16.65 500
        500 http://ports.ubuntu.com/ubuntu-ports bionic-updates/main arm64 Packages
        500 http://ports.ubuntu.com/ubuntu-ports bionic-security/main arm64 Packages
        100 /var/lib/dpkg/status