Create Downgrade.yml

[JoshuaLampert]

See trixi-framework/Trixi.jl#1771 and trixi-framework/P4est.jl#105.

[ranocha]

Thanks!

[JoshuaLampert]

I fixed the lower compat bounds such that the Downgrade action passes. In addition, I cleaned up the Project.toml a bit, i.e.:

1. I added compat bounds for stdlibs (Libdl, UUIDs, Test) as recommended in https://discourse.julialang.org/t/psa-compat-requirements-in-the-general-registry-are-changing.
2. I removed TOML from the Project.toml since it is not used in T8code.jl.

[JoshuaLampert]

Ok to merge, @jmark?