Don't duplicate matrix in github actions file

[fabienheureux]

Following this comment #152 (comment)

I would like to suggest using a more synthetic approach to the way the matrix is written in https://github.com/torchbox/wagtail-wordpress-import/blob/main/.github/workflows/test.yml#L17-L20

We can easily use a json file in order to specify the versions, based on this example : https://github.com/nobe4/test-action-matrix
Let me know what you think of this approach, I would be happy to implement it 👍

[nickmoreton]

Closing as this was merged