SUPPORT.md: Argo: Upgrade status to Tech Preview

Recent patches to xen-devel indicate active interest in Argo within the Xen
community, and as the feature has been documented and in use by
OpenXT for multiple years, update the Xen SUPPORT.md statement of status.

This has also been discussed at several XenSummits, with general agreement to
the status upgrade.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

SUPPORT.md

@@ -848,7 +848,7 @@ This feature is not security supported: see https://xenbits.xen.org/xsa/advisory
 
 ### Argo: Inter-domain message delivery by hypercall
 
-    Status: Experimental
+    Status: Tech Preview
 
 ### x86/PCI Device Passthrough