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).

(From meta-google rev: 98b0607cb298193ee7a8033e0c333224a10d0477)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie038d9ad9f7f785fd8b3091cb19d6b593924eace
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/meta-google/conf/layer.conf b/meta-google/conf/layer.conf
index 03f9c68..a8fda28 100644
--- a/meta-google/conf/layer.conf
+++ b/meta-google/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"