Transition to KiCad 6

[apcountryman]

Transition to KiCad 6.

• [x] Update prototype-kicad-library-ci submodule
• [x] Update utility submodules
  • [x] utilities/kicad-library-utils (switch to master branch)
  • [x] utilities/KiAuto
  • [x] utilities/KiBoM
  • [x] utilities/InteractiveHtmlBom
  • [x] utilities/KiBot
  • [x] utilities/PcbDraw
• [x] Remove KiCad 5 project files scrub filter and CI job
  • [x] Remove .gitattributes (only contains KiCad 5 project file scrub filter)
  • [x] Remove git/config/install (only installs the KiCad 5 project file scrub filter)
  • [x] Remove README.md Git Config section
  • [x] Remove ci/scrub-kicad-project-files
  • [x] Remove .github/workflows/ci.yml scrub-kicad-project-files CI job
• [x] Update symbols KLC compliance CI script
  • [x] Symbol library file extension changed from .lib to .kicad_sym
  • [x] KiCad utility script changed from utilities/kicad-library-utils/schlib/checklib.py to utilities/kicad-library-utils/klc-check/check_symbol.py
• [x] Update footprints KLC compliance CI script
  • [x] KiCad utility script changed from utilities/kicad-library-utils/pcb/check_kicad_mod.py to utilities/kicad-library-utils/klc-check/check_footprint.py