Add compatibility file for GRUB versions up to v2.06

GRUB is not able to detect ZFS pool if snaphsot of top level boot
pool is created. This issue is observed with GRUB versions up to
v2.06 if extensible_dataset feature is enabled on ZFS boot pool.

compatibility=grub2-2.06 would enable all read-only compatible
zpool features except extensible_dataset and other features that
depend on it.

The existing grub2 compatibility file is now renamed to grub2-2.12 to
reflect the appropriate grub2 version. grub2-2.12 lists all read-only
features that can be enabled on boot pool for grub2 with version 2.12
onwards.

A new symlink grub2 is created that currently points to the grub2-2.12
compatibility file.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #13873
Closes #15261
Closes #15909