meta-ibm: 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: Ie8f086b570be48521ab2342392c21539c0e2d6bc
diff --git a/conf/layer.conf b/conf/layer.conf
index 1796ae6..8662448 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -7,4 +7,4 @@
 
 BBFILE_COLLECTIONS += "ibm-layer"
 BBFILE_PATTERN_ibm-layer := "^${LAYERDIR}/"
-LAYERSERIES_COMPAT_ibm-layer = "zeus dunfell gatesgarth"
+LAYERSERIES_COMPAT_ibm-layer = "dunfell gatesgarth"