Make min_max_speed_davis default wave speed estimate for FluxHLL()

[DanielDoehring]

See #1721 and #1726

In this version, we keep min_max_speed_naive.

Closes #1721

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4369c1c) 96.38% compared to head (1ecce03) 95.96%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1743      +/-   ##
==========================================
- Coverage   96.38%   95.96%   -0.42%     
==========================================
  Files         455      455              
  Lines       36391    36391              
==========================================
- Hits        35074    34921     -153     
- Misses       1317     1470     +153     

Flag	Coverage Δ
unittests	95.96% <100.00%> (-0.42%)	⬇️

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[DanielDoehring]

The changes in the test directory stem from those elixirs for which the speed estimates have been exchanged.

[ranocha]

Is it necessary to merge main into this PR so often? If not, please refrain from doing so since it notifies people on GitHub and creates unnecessary friction since there isn't anything new.