Fix MPI tests on Apple Silicon

Not everything works on ARM:

> LoadError: User-defined reduction operators are currently not supported on non-Intel architectures.
See https://github.com/JuliaParallel/MPI.jl/issues/404 for more details.

see https://github.com/trixi-framework/Trixi.jl/actions/runs/8865649949/job/24342153523?pr=1562#step:7:1124

We need to update our CI settings to reflect this - can we still use older versions/architectures for some MacOS tests?

_Originally posted by @ranocha in https://github.com/trixi-framework/Trixi.jl/issues/1562#issuecomment-2081423259_

Start with the linked GitHub Actions run and the MPI.jl issue to confirm which Apple Silicon test fails and why. Inspect the repository's CI settings and test matrix; done means the matrix reflects supported macOS architectures or versions and the MPI tests pass on the configurations retained.

Written by the indexing model from the issue text.