Update RHCOS to OCP version map (#3236)

Co-authored-by: sebrandon1 <[email protected]>

Author: github-actions[bot]

tests/platform/operatingsystem/files/rhcos_version_map

@@ -630,6 +630,7 @@
 4.20.0-rc.0 / 9.6.20250826-1
 4.20.0-rc.1 / 9.6.20250908-0
 4.20.0-rc.2 / 9.6.20250909-0
+4.21.0-ec.0 / 9.6.20250904-0
 4.4.0 / 44.81.202004260825-0
 4.4.0-rc.0 / 44.81.202003110830-0
 4.4.0-rc.1 / 44.81.202003130330-0