Update RHCOS to OCP version map (#3179)

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

Author: github-actions[bot]

tests/platform/operatingsystem/files/rhcos_version_map

@@ -615,6 +615,7 @@
 4.20.0-ec.3 / 9.6.20250611-0
 4.20.0-ec.4 / 9.6.20250707-1
 4.20.0-ec.5 / 9.6.20250726-0
+4.20.0-ec.6 / 9.6.20250815-0
 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