Update rn-kubernetes-installer.md

Author: nvanthao

docs/release-notes/rn-kubernetes-installer.md

@@ -10,6 +10,12 @@ This topic contains release notes for the [Replicated kURL](/vendor/kurl-about)
 
 <!--RELEASE_NOTES_PLACEHOLDER-->
 
+## v2025.07.21-0
+
+Released on July 21, 2025
+
+### New Features {#new-features-v2025-07-21-0}
+* Adds support for RHEL 9.6 and Rocky Linux 9.6.
 
 
 ## v2025.07.09-0