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 dunfell (previous).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2a1239a7373faad7732f010b3baded09921daa73
diff --git a/meta-palmetto/conf/layer.conf b/meta-palmetto/conf/layer.conf
index 895e53b..724670d 100644
--- a/meta-palmetto/conf/layer.conf
+++ b/meta-palmetto/conf/layer.conf
@@ -7,4 +7,4 @@
 
 BBFILE_COLLECTIONS += "palmetto-layer"
 BBFILE_PATTERN_palmetto-layer := "^${LAYERDIR}/"
-LAYERSERIES_COMPAT_palmetto-layer = "zeus dunfell gatesgarth"
+LAYERSERIES_COMPAT_palmetto-layer = "dunfell gatesgarth"
diff --git a/meta-romulus/conf/layer.conf b/meta-romulus/conf/layer.conf
index 4288845..4ca0ead 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 = "zeus dunfell gatesgarth"
+LAYERSERIES_COMPAT_romulus-layer = "dunfell gatesgarth"