image-defaults: add note about when to drop option

squashfs-compression: zstd will become obsolete soon. Let's drop
a note about it so anyone happening by can clean it up.

src/image-default.yaml

@@ -21,6 +21,7 @@ deploy-via-container: false
 # container-imgref: ""
 
 # Format used when generating a squashfs image.  Can also be e.g. gzip or lz4
+# XXX: Legacy. Delete this when buildextend-live is either deleted or becomes a symlink.
 squashfs-compression: zstd
 
 # Defaults for VMware OVA, matching historical behavior