netplan should specify the version of YAML that it takes as input

[Dan Watkins]

In YAML 1.1, some MAC addresses can be interpreted as base 60 integers (per https://yaml.org/type/int.html) whereas YAML 1.2 dropped support for base 60 integers.

For people generating config programmatically, this could lead to issues if they are using a YAML 1.2 generator (and assuming netplan uses 1.1): their library will not quote such base 60 integers (because in YAML 1.2 they are unambiguously strings), and so a YAML 1.1 library will parse that value as an integer instead of a string.

PyYAML (`import yaml`) is YAML 1.1 where as `ruamel.yaml` is YAML 1.2:

```
In [1]: import yaml
In [2]: import ruamel.yaml

# The value roundtrips correctly if only 1.1 is used
In [4]: yaml.safe_load(yaml.dump({"x": "11:22:33:44:55:59"}))
Out[4]: {'x': '11:22:33:44:55:59'}

# The value roundtrips correctly if only 1.2 is used
In [6]: ruamel.yaml.safe_load(ruamel.yaml.dump({"x": "11:22:33:44:55:59"}))
Out[6]: {'x': '11:22:33:44:55:59'}

# The value is interpreted as a base60 integer if 1.2 is the dumper and 1.1 is the loader
In [8]: yaml.safe_load(ruamel.yaml.dump({"x": "11:22:33:44:55:59"}))
Out[8]: {'x': 8846009759}
```

The key difference is in the quoting:

```
In [10]: yaml.dump({"x": "11:22:33:44:55:59"})
Out[10]: "x: '11:22:33:44:55:59'\n"

In [11]: ruamel.yaml.dump({"x": "11:22:33:44:55:59"})
Out[11]: '{x: 11:22:33:44:55:59}\n'
```