Update netboot.xyz.lkrn to v2.0.76, properly supporting Grml 2022.11 release

Update netboot.xyz.lkrn to v2.0.76, properly supporting Grml 2022.11 release

Retrieved via:

  % cd templates/boot/addons/
  % wget -q https://github.com/netbootxyz/netboot.xyz/releases/download/2.0.76/netboot.xyz.efi
  % wget -q https://github.com/netbootxyz/netboot.xyz/releases/download/2.0.76/netboot.xyz.lkrn
  % wget -q https://github.com/netbootxyz/netboot.xyz/releases/download/2.0.76/netboot.xyz-sha256-checksums.txt
  % sha256sum --ignore-missing -c netboot.xyz-sha256-checksums.txt
  netboot.xyz.efi: OK
  netboot.xyz.lkrn: OK
  sha256sum: WARNING: 1 line is improperly formatted

As of release 2.0.73, Grml 2022.11 works again, see
https://github.com/netbootxyz/netboot.xyz/issues/1310