SUPPORT.md: mark xenstore live update as supported

Live update of xenstored is available since Xen 4.15 and it is tested
on a regular basis since then.

Switch the live update support from "Tech Preview" to "Supported".

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

SUPPORT.md

@@ -270,7 +270,7 @@ or itself will not be regarded a security issue.
 ### C xenstored daemon
 
     Status: Supported
-    Status, Liveupdate: Tech Preview
+    Status, Liveupdate: Supported
 
 ### C xenstore stubdom PV