sysutils/memtest86+: deprecate the port and set expiration date

The crown jewel of the port, an ELF binary that could be booted by
FreeBSD BIOS boot-chain, has been broken for a long while now.
Besides, most systems use EFI boot now and the EFI binary available from
the upstream can be used on those systems.
Also, many systems based on coreboot provide built-in memtest86+.
So, the port no longer serves any useful purpose.