meta-facebook: update Yocto compat for gatesgarth

The next release of Yocto is soon and most of the upstream
layers have switched support strings for it.  Support layer
compat for gatesgarth (current) and dunfell (previous).

(From meta-facebook rev: 14fc581f37beea18d5b1e6eb04fbb014be5cd357)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5c460251ca3e1f7cd2a48008238cd7885e44493d
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>