convert two reboot tasks running on changed to handlers

This fixes ansible-lint no-handler complaints.

playbooks/roles/apt_update_and_upgrade/handlers/main.yaml

@@ -0,0 +1,3 @@
+- name: reboot the system
+  become: true
+  ansible.builtin.reboot:

playbooks/roles/apt_update_and_upgrade/tasks/main.yaml

@@ -9,7 +9,5 @@
       ansible.builtin.apt:
         upgrade: dist
       register: apt_update_and_upgrade__upgrade_result
-
-    - name: reboot, after package upgrade
-      ansible.builtin.reboot:
-      when: apt_update_and_upgrade__upgrade_result.changed
+      notify:
+        - reboot the system