Rename draft-THREAT-MODEL.md to THREAT_MODEL.md (#13599)

The project-wide security threat model merged (#13293)
as draft-THREAT-MODEL.md, but the canonical discoverability name that
scanners and satellite-repo SECURITY.md pointers follow is THREAT_MODEL.md.

This renames the file to THREAT_MODEL.md and updates the in-repo
SECURITY.md reference, making the pointer in apache/cloudstack-cloudmonkey
(which already targets .../blob/main/THREAT_MODEL.md) resolve. The
document's own review-status wording is unchanged.

Generated-by: Claude Opus 4.8 (1M context)

Author: potiuk

SECURITY.md

@@ -31,4 +31,4 @@ For more details, see https://cloudstack.apache.org/security.html.
 What the project treats as in scope and out of scope, the security
 properties it provides and disclaims, the adversary model, and how
 findings are triaged are documented in the project-wide threat model:
-[draft-THREAT-MODEL.md](draft-THREAT-MODEL.md).
+[THREAT_MODEL.md](THREAT_MODEL.md).