SUPPORT.md: change current version to 4.19-unstable

The version in SUPPORT.md is still "4.18-rc".

Change it to "4.19-unstable".

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

SUPPORT.md

@@ -9,7 +9,7 @@ for the definitions of the support status levels etc.
 
 # Release Support
 
-    Xen-Version: 4.18-rc
+    Xen-Version: 4.19-unstable
     Initial-Release: n/a
     Supported-Until: TBD
     Security-Support-Until: Unreleased - not yet security-supported