Rename midplane interface eth1-midplane

As per discussions with Microsoft and sonic-buildimage PR8583
(sonic-net/sonic-buildimage#8583), we are
renaming the midplane interface from eth0.$VLAN_ID to eth1-midplane.

Change-Id: I2ae2188611a8018fc8d81ad0b3c3e1e58d6a48c3

utils/lc-interface-config

@@ -74,7 +74,7 @@ do_supervisor_mode() {
 }
 
 do_linecard_mode() {
-   local vlan_intf="eth0.$VLAN_ID"
+   local vlan_intf="eth1-midplane"
 
    echo "Setting up $vlan_intf"