Ubuntu changed package name python-apt to python3-apt (#7769)

* replaced deprecated python package with python3 package

* removed the version due to duplication

roles/kubernetes/preinstall/vars/ubuntu-21.04.yml

----
-required_pkgs:
-  - python3-apt
-  - aufs-tools
-  - apt-transport-https
-  - software-properties-common
-  - conntrack

roles/kubernetes/preinstall/vars/ubuntu.yml

 ---
 required_pkgs:
-  - python-apt
+  - python3-apt
   - aufs-tools
   - apt-transport-https
   - software-properties-common