refactor: use the yml extension for every yaml file

The repository mixed both extensions, the molecule scenario used yml while the role itself used yaml. The yml
extension is the one ansible-galaxy and molecule generate and the one the sibling roles use, so it is applied
throughout.

The first_found lookups in tasks/main.yml name the candidate files explicitly and were adjusted accordingly, otherwise
the role would no longer find its distribution variables and its verification tasks.

Co-authored-by: Copilot <copilot@github.com>

tasks/verify_vars.yml

@@ -0,0 +1,12 @@
+---
+
+- name: Verify that each entry defines either a user or a group
+  ansible.builtin.assert:
+    that:
+    - (item.user is defined and item.user | length > 0) !=
+      (item.group is defined and item.group | length > 0)
+    fail_msg: >
+      Each entry of sudo_users_sudoers must define either 'user' or 'group',
+      but not both and not none.
+    quiet: true
+  with_items: "{{ sudo_users_sudoers }}"