Update contrib/initramfs/README.initramfs.markdown

Note that Dropbear supports ed25519 keys since version 2020.79.

See https://github.com/mkj/dropbear/pull/91

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Michael Franzl <michael@franzl.name>
Closes #12715