meta-microsoft: 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 dunsfell (previous).

(From meta-microsoft rev: 8d91440d4c9f86b156694f2870505f71f50d6441)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ieb116a883c74f174fe65cd74b61da34eaf109554
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>