remove n_cells_max argument from TreeMesh constructor

[ranocha]

This is a follow-up to #3010; here, I removed n_cells_max completely (as a breaking change).

This PR was created with the help of AI tools but supervised and reviewed by me.

TODO

• Update Trixi2Vtk.jl
• Update TrixiShallowWater.jl

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.10%. Comparing base (4eb6a6b) to head (fe0f791).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3021   +/-   ##
=======================================
  Coverage   97.10%   97.10%           
=======================================
  Files         630      630           
  Lines       48851    48825   -26     
=======================================
- Hits        47434    47411   -23     
+ Misses       1417     1414    -3     

Flag	Coverage Δ
unittests	97.10% <100.00%> (+<0.01%)	⬆️

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