Fix linter warnings

[sengels-tum]

Description

This PR fixes all linter warnings due to an updated version of cpp-linter.

P.S.: Moreover, it changes CTEST_PARALLEL_LEVEL to 4, because the Gurobi licensing issues seem to have been caused by multiple CI jobs running in parallel and not directly by google tests running in parallel.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 98.27586% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.3%. Comparing base (7af816e) to head (fc399d1).

Files with missing lines	Patch %	Lines
src/solver/mip-based/GenPOMovingBlockMIPSolver.cpp	75.0%	2 Missing ⚠️
...olver/mip-based/GenPOMovingBlockMIPSolver_Lazy.cpp	90.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #105     +/-   ##
=======================================
- Coverage   97.3%   97.3%   -0.1%     
=======================================
  Files         44      44             
  Lines      13108   13060     -48     
  Branches    1571    1559     -12     
=======================================
- Hits       12758   12708     -50     
- Misses       350     352      +2     

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