fix: 26.3 release note for OpenShift (#836)

Author: razvan

modules/ROOT/partials/release-notes/release-26.3.adoc

@@ -491,6 +491,7 @@ These Kubernetes versions are no longer supported:
 
 This release is available in the RedHat Certified Operator Catalog for the following OpenShift versions:
 
+* `4.21`
 * `4.20`
 * `4.19`
 * `4.18`