Fix stage3 issues: obsolete k8s deb packages and mongodb charm version

The change updates LCM Dockerfile and OSM installer to use Kubernetes
version 1.28 instead of 1.26.

In addition, mongodb is installed from 6/stable channel instead of
latest/stable

Change-Id: Ib67654765ddec0990af70404f7f8a7fff68bd1ef
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/installers/install_kubeadm_cluster.sh b/installers/install_kubeadm_cluster.sh
index 108a8ea..40c9527 100755
--- a/installers/install_kubeadm_cluster.sh
+++ b/installers/install_kubeadm_cluster.sh
@@ -15,8 +15,8 @@
 
 set +eux
 
-K8S_VERSION=1.26
-K8S_PACKAGE_VERSION="$K8S_VERSION".15-1.1
+K8S_VERSION=1.28
+K8S_PACKAGE_VERSION="$K8S_VERSION".2-1.1
 
 # installs kubernetes packages
 function install_kube() {