Calico throws an error when setting a `veth-mtu` value

[Mateo Florido]

The charm fails when setting a value for the `veth-mtu` configuration value.

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-calico-0/charm/venv/ops/manifests/manifest.py", line 313, in apply_resources
    self.client.apply(rsc.resource, force=True)
  File "/var/lib/juju/agents/unit-calico-0/charm/venv/lightkube/core/client.py", line 457, in apply
    return self.patch(type(obj), name, obj, namespace=namespace,
  File "/var/lib/juju/agents/unit-calico-0/charm/venv/lightkube/core/client.py", line 325, in patch
    return self._client.request("patch", res=res, name=name, namespace=namespace, obj=obj,
  File "/var/lib/juju/agents/unit-calico-0/charm/venv/lightkube/core/generic_client.py", line 245, in request
    return self.handle_response(method, resp, br)
  File "/var/lib/juju/agents/unit-calico-0/charm/venv/lightkube/core/generic_client.py", line 196, in handle_response
    self.raise_for_status(resp)
  File "/var/lib/juju/agents/unit-calico-0/charm/venv/lightkube/core/generic_client.py", line 190, in raise_for_status
    raise transform_exception(e)
lightkube.core.exceptions.ApiError: failed to create typed patch object (kube-system/calico-config; /v1, Kind=ConfigMap): .data.veth_mtu: expected string, got &value.valueUnstructured{Value:8750}

[Mateo Florido (mateoflorido)]

[PR](https://github.com/charmed-kubernetes/charm-calico/pull/103)

[Adam Dyess (addyess)]

Backported to release_1.28 branch