Convert KiCad schematics and projects to version 6

[ohmtech-rdi]

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Since version 6, KiCad has been adopting the same s-expression format for its schematics as the rest of its file formats, as the one for the PCB layout & routing, and moved to a JSON representation for its project file.

For now, all schematics and projects checked-in the repository are still in Version 5.

This doesn't cause any direct problems, but is not ideal when creating a new control from an existing control.

In particular, the intermediate .net file is no longer used, and our PCB generation depends on it. For this, probably remove the kcgen dependency.

• Update all schematics and projects (files finishing with .sch and .pro) to version 6 (kicad_sch and kicad_pro),
• Update all PCB files to version 6 (kicad_pcb).