Kicad v6 upgrade

[alexanderwiller]

Following up to #32, I have made the technical conversion of the project to KiCad V6.

What has been done:

• The .gitignore file has been modified to exclude the right files for KiCad V6 file naming
• I used the library https://github.com/Aharoni-Lab/KiCad-Library that was made available and updated/created the fp-lib-table and sym-lib-table using the KICAD_AHARONI_LAB variable to properly reference these libraries
• I opened the project, schematic and PCB files with KiCad V6, verified correct loading of libraries (schematic is now intact, footprints can be updated from the libraries if desired) and saved the files again to do the upgrade
• After that, all old format files and some other random generated files were deleted
• Finally, I refilled the copper zones to avoid any surprises to future users and checked that the result is correct. To do this, I both did a before/after check in KiCad visually, also I created the following diff using KiDiff: Miniscope-v4-Rigid-Flex-diff_pcb.pdf
  Minor variations in the fill can be observed at the edges as coloured regions, but they are not functionally relevant at all and all clearances are still maintaned.

I chose to neither do a Gerber re-export nor to change the version number, as this is only a "background" technical update that effectively results in the same board, from a functional point of view.

When this PR is deemed ok and merged, next I would like to improve the footprint library components used in this board to include pin 1 markers and RefDes information on the assembly layer according to KLC, so that assembly is more straight-forward and risk for errors is reduced.