Set Kubernetes 1.28 as min required version

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/check.go

@@ -60,7 +60,7 @@ type checkFlags struct {
 }
 
 var kubernetesConstraints = []string{
-	">=1.26.0-0",
+	">=1.28.0-0",
 }
 
 var checkArgs checkFlags