Improvement: Remove support of gmsh mesh format 2

[spenke91]

closes #2071

Describe your changes here:

This is still a draft. There are two open issues I would like to discuss briefly in the next dev meeting (unless someone can answer them already, for example, @sandro-elsweijer?) :

1. The example example/IO/cmesh/gmsh/t8_load_and_refine_square_w_hole.cxx does not work on origin/main (unless I did something wrong). I updated the associated mesh file to version 4.1, but I cannot properly test it - except for confirming that the error message is still the same 😆 I can look into it but maybe it is a known issue?
2. The script su2_mesh_to_gmsh.py writes a gmsh file of mesh format version 2.2 as an output. Before I or someone looks into how to update it to version 4.1: Do we still need this script or is it some outdated functionality?

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