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 dunfell (previous).

(From meta-microsoft rev: 2518b2a71b8373ccfaa6015b88c76949244afb03)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I897ebf4e697b9ef21aa22462d3f1f1bb5e902804
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>