initramfs-tools: use upstream's xz handling to support multithreading

initramfs-tools: use upstream's xz handling to support multithreading

initramfs-tools versions >=0.136 support multithreading in
xz compression usage (see upstream commit 41d9ceb3).

While at it fix header comment, to refer to the actual
script (which was renamed in commit 6e778099).

FTR: we're still using xz (and not using e.g. zstd) because
it still seems to be the combination of small file size
*and* decompression speed now that we have multithreading
support (though zstd might be an interesting option in the future),
also see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=976054