Netplan not merging routes in separate files

[Husam Bilal]

Adding extra static routes in a separate configuration file doesn't seem to get merged.

Release info:
```
Distributor ID: Ubuntu
Description: Ubuntu 22.04 LTS
Release: 22.04
Codename: jammy
```

Example config files:
```
# /etc/netplan/00-config.yaml
network:
  version: 2
  ethernets:
    ens5:
      dhcp4: true
      dhcp6: false
      dhcp4-overrides:
        use-routes: false
      routes:
        - to: 172.31.0.0/16
          via: 172.31.0.1
```
```
# /etc/netplan/10-config.yaml
network:
  version: 2
  ethernets:
    ens5:
      routes:
        - to: 172.18.0.0/24
          via: 172.31.1.240
```

Running `netplan --debug get` returns the following output:
```
** (process:1249606): DEBUG: 00:42:14.001: starting new processing pass
** (process:1249606): DEBUG: 00:42:14.001: ens5: adding new route
** (process:1249606): DEBUG: 00:42:14.001: We have some netdefs, pass them through a final round of validation
** (process:1249606): DEBUG: 00:42:14.001: ens5: setting default backend to 1
** (process:1249606): DEBUG: 00:42:14.001: Configuration is valid
network:
  version: 2
  ethernets:
    ens5:
      dhcp4: true
      dhcp4-overrides:
        use-routes: false
      dhcp6: false
      routes:
      - to: "172.31.0.0/16"
        via: "172.31.0.1"
```