Pin Postgres version to 15.0

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2180

Just specifying `15` means we won't automatically re-pull `15.1` when it comes out.

roles/matrix-postgres/defaults/main.yml

@@ -29,7 +29,7 @@ matrix_postgres_docker_image_v11: "{{ matrix_container_global_registry_prefix }}
 matrix_postgres_docker_image_v12: "{{ matrix_container_global_registry_prefix }}postgres:12.12{{ matrix_postgres_docker_image_suffix }}"
 matrix_postgres_docker_image_v13: "{{ matrix_container_global_registry_prefix }}postgres:13.8{{ matrix_postgres_docker_image_suffix }}"
 matrix_postgres_docker_image_v14: "{{ matrix_container_global_registry_prefix }}postgres:14.5{{ matrix_postgres_docker_image_suffix }}"
-matrix_postgres_docker_image_v15: "{{ matrix_container_global_registry_prefix }}postgres:15{{ matrix_postgres_docker_image_suffix }}"
+matrix_postgres_docker_image_v15: "{{ matrix_container_global_registry_prefix }}postgres:15.0{{ matrix_postgres_docker_image_suffix }}"
 matrix_postgres_docker_image_latest: "{{ matrix_postgres_docker_image_v15 }}"
 
 # This variable is assigned at runtime. Overriding its value has no effect.