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

cmd/flux/testdata/check/check_pre.golden

@@ -1,3 +1,3 @@
 ► checking prerequisites
-✔ Kubernetes {{ .serverVersion }} >=1.26.0-0
+✔ Kubernetes {{ .serverVersion }} >=1.28.0-0
 ✔ prerequisites checks passed