repo: Rename compose templates to compose.yaml.jinja

It is a short, concicse name for the compose (and one of the ways that
docker officially recommends to name the files), and jinja is the
careful recommendation of the jinja docs.

https://jinja.palletsprojects.com/en/stable/templates/#template-file-extension

roles/grocy/tasks/main.yaml

@@ -10,7 +10,7 @@
 - name: Start the compose stack
   community.docker.docker_compose_v2:
     project_name: stack_grocy
-    definition: "{{ lookup('template', 'docker-compose.yaml.j2') | from_yaml }}"
+    definition: "{{ lookup('template', 'compose.yaml.jinja') | from_yaml }}"
     remove_orphans: true
     wait: true
     wait_timeout: 60