update alertmanager version in enterprise_versions.yml (#3738)

Author: ti-afra

config/enterprise_versions.yml

@@ -110,7 +110,7 @@ components:
   # coreos-prometheus holds the version of alertmanager built for tigera/alertmanager,
   # which prometheus operator uses to validate.
   coreos-alertmanager:
-    version: v0.27.0
+    version: v0.28.0
   alertmanager:
     image: tigera/alertmanager
     version: master