meta-phosphor: 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).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia4dea5e8e713677288e2fdf25d2293d9a5f35b11
diff --git a/conf/layer.conf b/conf/layer.conf
index 028c6e5..2292acc 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -8,7 +8,7 @@
 BBFILE_COLLECTIONS += "phosphor-layer"
 BBFILE_PATTERN_phosphor-layer := "^${LAYERDIR}/"
 LAYERVERSION_phosphor-layer = "1"
-LAYERSERIES_COMPAT_phosphor-layer = "warrior zeus dunfell"
+LAYERSERIES_COMPAT_phosphor-layer = "dunfell gatesgarth"
 
 IMAGE_FEATURES[validitems] += "tools-profile"