Documentation: Mark the 'efivars' sysfs interface as removed

The 'efivars' sysfs interface was removed in commit 0f5b2c6 ("efi:
vars: Remove deprecated 'efivars' sysfs interface"), but the ABI
documentation was not updated properly.

Strip down the documentation file for /sys/firmware/efi/vars to a very
basic description of what the interface was about, add a section about
the rough removal timeline, and inform the reader about the intended
replacement.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Author: timschumi

Documentation/ABI/removed/sysfs-firmware-efi-vars

@@ -0,0 +1,12 @@
+What:		/sys/firmware/efi/vars
+Date:		April 2004, removed March 2023
+Description:
+		This directory exposed interfaces for interacting with
+		EFI variables.  For more information on EFI variables,
+		see 'Variable Services' in the UEFI specification
+		(section 7.2 in specification version 2.3 Errata D).
+
+		The 'efivars' sysfs interface was removed in March of 2023,
+		after being considered deprecated no later than September
+		of 2020. Its functionality has been replaced by the
+		'efivarfs' filesystem.

Documentation/ABI/stable/sysfs-firmware-efi-vars

@@ -40,4 +40,4 @@ accidentally.
 *See also:*
 
 - Documentation/admin-guide/acpi/ssdt-overlays.rst
-- Documentation/ABI/stable/sysfs-firmware-efi-vars
+- Documentation/ABI/removed/sysfs-firmware-efi-vars