Add support for Windows 2019 series

[Simon Richardson]

In the package juju/os, we need to bump the windows version to also include Windows 2019. As part of this we also need to add a job/bot to verify that the windows tests are correct. At the moment they _seem_ to be broken by the changes applied to ReadSeries (renamed).

The reason for the change is to ensure we can use github actions to run the juju/client tests in a windows container. The windows container only allows running with Windows 2019 and because we don't support that yet in juju/os, we panic when running the test[1].

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There is a preliminary PR[2] to add the new series, but the bot hasn't been completed yet.

 [1] https://github.com/juju/juju/pull/10668/checks?check_run_id=242255394#step:10:18
 [2] https://github.com/juju/os/pull/13