add compat bounds for stdlibs

[ranocha]

I added compat bounds for stdlibs since we cannot register new versions of Trixi.jl without them. I followed the recommendation in https://discourse.julialang.org/t/psa-compat-requirements-in-the-general-registry-are-changing/104958 since we didn't get any response in JuliaRegistries/TagBot#292.

The only exception to the rule above is Downloads - which is available in Julia v1.8 only as v1.6 and not v1.8 as usual...

Closes #1693, closes #1694

[github-actions]

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[sloede]

Note that they recently updated the PSA and are now (IMHO) somewhat confusingly also recommend to set the compat bound to 1 for packages supporting Julia 1.4 or higher. Since CompatHelper still wants to set bounds to 1, maybe we should do so as well for the time being at least?

[ranocha]

Done

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3186df5) 83.00% compared to head (ed2bfc9) 72.60%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1711       +/-   ##
===========================================
- Coverage   83.00%   72.60%   -10.40%     
===========================================
  Files         431      431               
  Lines       34665    34637       -28     
===========================================
- Hits        28772    25145     -3627     
- Misses       5893     9492     +3599     

Flag	Coverage Δ
unittests	72.60% <ø> (-10.40%)	⬇️

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see 116 files with indirect coverage changes

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[sloede]

Thanks!