CompatHelper: bump compat for MPI to 0.20, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the MPI package from ^0.19 to ^0.19, 0.20. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (72d6097) 20.05% compared to head (cd10140) 20.05%.

Additional details and impacted files

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[amontoison]

I propose to drop MPI 0.19.x.

--> Breaking changes with v0.20: The way how MPI.jl is configured to use different MPI implementations has changed from v0.19 to v0.20 in a non-backward-compatible manner. Specifically, most JULIA_MPI_XXX variables do not have an effect anymore. Please refer to the docs for information on how to migrate your existing configuration.