Use julia-downgrade-compat@v2

[JoshuaLampert]

With the new julia-downgrade-compat v2, the action switched to use Resolver.jl to find minimal compatible versions. If everything works as intended, this makes it MUCH simpler to work with and debug the downgrade action because in case of incompatible lower bounds, you don't need to resolve the incompatibilities manually by finding out a set of working minimal bounds. This should now be done by Resolver.jl.

[github-actions]

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[JoshuaLampert]

We also run into StefanKarpinski/Resolver.jl#15. DiffEqBase.jl v6.155.2 and v6.155.3 are yanked, but Resolver.jl doesn't realize that and wants to take v6.155.2.

[JoshuaLampert]

And we still get some of the Unsatisfiable requirements detected errors. I thought Resolver.jl would be smart enough to get around these. I thought that's actually the whole point, but either I'm misunderstanding something or there are still bugs in Resolver.jl... 😬

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.68%. Comparing base (035aec0) to head (07a59b2).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2507   +/-   ##
=======================================
  Coverage   96.68%   96.68%           
=======================================
  Files         511      512    +1     
  Lines       42278    42300   +22     
=======================================
+ Hits        40875    40897   +22     
  Misses       1403     1403           

Flag	Coverage Δ
unittests	96.68% <ø> (+<0.01%)	⬆️

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