graphics/intel-backlight: Deprecate the port

Now that FreeBSD 12.X is EoLed every FreeBSD version have the backlight
subsystem. backlight(8) works with any Intel or AMD hardware as long as drm-kmod
is loaded.

Differential Review:	https://reviews.freebsd.org/D44194
Approved by:		maintainer timeout (1 month)
Sponsored by:		Beckhoff Automation GmbH & Co. KG

graphics/intel-backlight/Makefile

@@ -1,6 +1,6 @@
 PORTNAME=		intel-backlight
 DISTVERSION=		20200418
-PORTREVISION=		2
+PORTREVISION=		3
 CATEGORIES=		graphics
 
 MAINTAINER=		jmd@FreeBSD.org
@@ -10,6 +10,9 @@ WWW=		https://github.com/FreeBSDDesktop/intel_backlight_fbsd
 LICENSE=		MIT
 LICENSE_FILE=		${WRKSRC}/COPYING
 
+DEPRECATED=		Use backlight(8)
+EXPIRATION_DATE=	20240701
+
 ONLY_FOR_ARCHS=		amd64 i386
 ONLY_FOR_ARCHS_REASON=	this hardware is only found on Intel x86 systems