initrd: mark initrd support as deprecated

The classic initial ramdisk has been replaced by the much more
flexible and efficient initramfs a long time.  Warn about it being
removed soon.

Includes a spelling fix from Colin Ian King <colin.king@canonical.com>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>