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).

(From meta-ibm rev: 00e954d3100bb932d154273376620b40d6b8cad3)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iffd47517e7b9e85597fe97478375a48d0c2db927
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/meta-ibm/conf/layer.conf b/meta-ibm/conf/layer.conf
index 1796ae6..8662448 100644
--- a/meta-ibm/conf/layer.conf
+++ b/meta-ibm/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"