SUPPORT.md: update version to 4.18-unstable

The current version is no longer 4.17-rc. Update it to "4.18-unstable".

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

SUPPORT.md

@@ -9,7 +9,7 @@ for the definitions of the support status levels etc.
 
 # Release Support
 
-    Xen-Version: 4.17-rc
+    Xen-Version: 4.18-unstable
     Initial-Release: n/a
     Supported-Until: TBD
     Security-Support-Until: Unreleased - not yet security-supported