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, so it is applied throughout and across the sibling roles.

The include_tasks calls in tasks/main.yml and tasks/create_unix_user.yml name their target explicitly and were
adjusted accordingly, otherwise the role would no longer find the included task files.

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

tasks/create_unix_group.yml

@@ -0,0 +1,8 @@
+---
+
+- name: "Create unix group: {{ unix_group.key }}"
+  ansible.builtin.group:
+    name: "{{ unix_group.key }}"
+    # The boolean form of default also omits an empty gid, which means let the system assign one.
+    gid: "{{ unix_group.value.gid | default(omit, true) }}"
+    state: "{{ unix_group.value.state | default('present') }}"