gatesgarth: add support for new series

As we add support for a new series, remove support for the oldest.
Little to no testing is done with older ones.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I24b73d4b3cbeceeb59493c367ef09d75f245b921
diff --git a/meta-romulus/conf/layer.conf b/meta-romulus/conf/layer.conf
index fa2e2af..4288845 100644
--- a/meta-romulus/conf/layer.conf
+++ b/meta-romulus/conf/layer.conf
@@ -7,4 +7,4 @@
 
 BBFILE_COLLECTIONS += "romulus-layer"
 BBFILE_PATTERN_romulus-layer := "^${LAYERDIR}/"
-LAYERSERIES_COMPAT_romulus-layer = "warrior zeus dunfell"
+LAYERSERIES_COMPAT_romulus-layer = "zeus dunfell gatesgarth"