Update CI configuration to run tests on driver v1.21 and stable (v2)

Now that the driver v2 have been release, the "stable" version of the `mongodb` extension is `v2.0.0+`.
We need to rework the CI configuration to run tests on the latest v1.x (v1.21) and the latest v2.x (stable).

https://github.com/mongodb-labs/symfony-mongodb/blob/83b17bce4086aba11d24aca4aefdb4ee2b68a931/.github/workflows/ci.yaml#L63