feat(privatek8s): upgrade to kubernetes 1.28 (#756)

as per
jenkins-infra/helpdesk#4144 (comment)

upgrading the privatek8s kubernetes cluster and node pools to 1.28.9

Signed-off-by: jayfranco999 <jaideepfrancis999@gmail.com>
Co-authored-by: Stéphane MERLE <95630726+smerle33@users.noreply.github.com>

locals.tf

@@ -49,7 +49,7 @@ locals {
   kubernetes_versions = {
     "cijenkinsio_agents_1"      = "1.28.9"
     "infracijenkinsio_agents_1" = "1.28.9"
-    "privatek8s"                = "1.27.9"
+    "privatek8s"                = "1.28.9"
     "publick8s"                 = "1.27.9"
   }