Subiquity ignores the IPv4 default gateway and creates network settings incorrectly.

[Kensuke Yanai]

I prepared the 22.04.2 ISO file to install Ubuntu Server. I loaded the image and launched the installer. Since there is no DHCP server in that network environment, I manually configured the IPv4 and IPv6 network settings. After proceeding with the steps, I tried to fetch the SSH Key from GitHub in the "SSH Setup" section, but encountered an error saying "Failed to establish a new connection: [Errno 101] Network is unreachable."

Upon investigating the cause, I found that the IPv4 gateway setting was missing in `/etc/netplan/00-installer-config.yaml`.

```yaml
# This is the network config written by 'subiquity'
network:
  ethernets:
    ens3:
      addresses:
      - ***.***.***.***/23
      - ****:****::****/64
      nameservers:
        addresses:
        - ***.****.***.***
        - ****:****::****
        search: []
      routes:
      - to: default
        via: fe80::1
  version: 2
```

In this situation, I tested the ping command:

```shell
root@ubuntu-server: /# ping 1.1.1.1
ping: connect: Network is unreachable

root@ubuntu-server: /# ping6 2606:4700:4700::1111
PING 2606:4700:4700::1111(2606:4700:4700::1111) 56 data bytes
64 bytes from 2606:4700:4700::1111: icmp_seq=1 ttl=57 time=1.41 ms
```

Next, I edited the /etc/netplan/00-installer-config.yaml file as follows:

```yaml
# This is the network config written by 'subiquity'
network:
  ethernets:
    ens3:
      addresses:
      - ***.***.***.***/23
      - ****:****::****/64
      nameservers:
        addresses:
        - ***.****.***.***
        - ****:****::****
        search: []
      routes:
      - to: default # here
        via: ***.***.***.*** # here
      - to: default
        via: fe80::1
  version: 2
```
Afterwards, by executing `sudo netplan apply`, I was able to connect to the IPv4 network as well.
I suspect that the netplan config generated by the installer is incorrect.

Can you please check and confirm this issue? Thank you for your cooperation.

---

I used the following ISO file:
https://releases.ubuntu.com/22.04.2/ubuntu-22.04.2-desktop-amd64.iso

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Uname: Linux 5.15.0-60-generic x86_64
Architecture: amd64
Date: Fri Mar 17 15:06:18 2023
ProcEnviron:
 TERM=linux
 PATH=(custom, no user)
 LANG=C.UTF-8
Snap: subiquity 23.02.1 ()
Snap.Changes:
 ID Status Spawn Ready Summary
 4 Done 2023-03-17T14:58:28Z 2023-03-17T14:58:28Z Switch "subiquity" snap to channel "stable/ubuntu-22.04.2"
Snap.Connections:

SnapSource: subiquity
Symptom: installer