meta-google: 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: Iae55ca0b666a271554175c7bfe34cf99b88a6db9
diff --git a/conf/layer.conf b/conf/layer.conf
index 03f9c68..a8fda28 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -7,4 +7,4 @@
 BBFILE_COLLECTIONS += "google-layer"
 BBFILE_PATTERN_google-layer := "^${LAYERDIR}/"
 LAYERVERSION_google-layer = "1"
-LAYERSERIES_COMPAT_google-layer = "warrior zeus dunfell"
+LAYERSERIES_COMPAT_google-layer = "dunfell gatesgarth"