Drop xslt/xsltproc from Kicad

Description

xslt is a very much legacy tech to transform xml into random output of all kinds. As a result libxslt is barely maintained at this point with outstanding bugs upstream. We don't have the ability or interest in maintaining libxslt (it's an entire language in of itself) and we have python for dynamic scripting to random output formats.

Let's slate this for v7.

• Announce the drop
• Remove libxslt from packaging (on Windows)
• Replace any stock xslt files in the kicad source with an equivalent
  • eeschema/plugins/xsl_scripts/bom2csv.xsl
  • eeschema/plugins/xsl_scripts/bom2grouped_csv.xsl
  • eeschema/plugins/xsl_scripts/netlist_form_cadstar.xsl
  • eeschema/plugins/xsl_scripts/netlist_form_OrcadPcb2.xsl
  • eeschema/plugins/xsl_scripts/netlist_form_cadstar-RINF.xsl
  • eeschema/plugins/xsl_scripts/netlist_form_pads-pcb.asc.xsl
  • eeschema/plugins/xsl_scripts/bom_with_title_block_2_csv.xsl

KiCad Version

Application: KiCad Schematic Editor (64-bit)

Version: (5.99.0-12165-gb0ba8b01c4), debug build

Libraries:
	wxWidgets 3.1.5
	libcurl/7.78.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19043), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Sep  1 2021 00:05:34
	wxWidgets: 3.1.5 (wchar_t,wx containers)
	Boost: 1.76.0
	OCC: 7.5.0
	Curl: 7.78.0-DEV
	ngspice: 35
	Compiler: Visual C++ 1929 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE_ADDRESS=OFF
	KICAD_SANITIZE_THREADS=OFF