meta-hpe: 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-hpe rev: 8a63411fb4d67d675f4ad3790ecbb1d2ec078bea)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I730e1762c251a9977814b2593850f32cc09b3377
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>