Create Downgrade.yml (second try)

[JoshuaLampert]

A new try for adding the downgrade CI, see #1771. The action also moved to https://github.com/julia-actions/julia-downgrade-compat. Skipping DiffEqBase is necessary because range specifiers in the Project.toml are not supported by the action. Once we have a proper fix for #1788, we should remove the skip again.

Closes #1771

[github-actions]

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c5e743a) 89.67% compared to head (cce9da9) 96.37%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1848      +/-   ##
==========================================
+ Coverage   89.67%   96.37%   +6.70%     
==========================================
  Files         458      458              
  Lines       36400    36411      +11     
==========================================
+ Hits        32640    35088    +2448     
+ Misses       3760     1323    -2437     

Flag	Coverage Δ
unittests	96.37% <ø> (+6.70%)	⬆️

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[ranocha]

LGTM. What do you think, @sloede ?

[sloede]

LGTM. Thanks a lot for adding this to Trixi.jl 👍

[ranocha]

Thanks!