SUPPORT.md: revive for extended security support "lifetime"

As per
https://lists.xen.org/archives/html/xen-devel/2026-02/msg00630.html.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

SUPPORT.md

@@ -12,7 +12,7 @@ for the definitions of the support status levels etc.
     Xen-Version: 4.17
     Initial-Release: 2022-12-12
     Supported-Until: 2024-06-12
-    Security-Support-Until: 2025-12-12
+    Security-Support-Until: 2027-12-12
 
 Release Notes
 : <a href="https://wiki.xenproject.org/wiki/Xen_Project_4.17_Release_Notes">RN</a>