Add note for upcoming ZoL 2.1 compatibility property

[jacobgreenleaf]

Makes a small note available both for existing users of bullseye-backports and the soon to be released bookworm release that the compatibility option is available and we can use grub2.

A future version of this guide referencing bookworm may want to continue enabling some of the features that are read-compatible but not on that list. That is:

zpool create \ 
    -o ashift=12 \ 
    -o autotrim=on -d \ 
    -o cachefile=/etc/zfs/zpool.cache \ 
-   -o feature@async_destroy=enabled \ 
-   -o feature@bookmarks=enabled \ 
-   -o feature@embedded_data=enabled \ 
-   -o feature@empty_bpobj=enabled \ 
-   -o feature@enabled_txg=enabled \ 
-   -o feature@extensible_dataset=enabled \ 
-   -o feature@filesystem_limits=enabled \ 
-   -o feature@hole_birth=enabled \ 
-   -o feature@large_blocks=enabled \ 
    -o feature@livelist=enabled \ 
-   -o feature@lz4_compress=enabled \ 
-   -o feature@spacemap_histogram=enabled \ 
    -o feature@zpool_checkpoint=enabled \ 
+   -o compatibility=grub2 \ 
    -O devices=off \ 
    -O acltype=posixacl -O xattr=sa \ 
    -O compression=lz4 \ 
    -O normalization=formD \ 
    -O relatime=on \ 
    -O canmount=off -O mountpoint=/boot -R /mnt \ 
    bpool 

And possibly device_rebuild, resilver_defer to be added.