Remove more hardcoded matrix-postgres references

roles/custom/matrix-postgres-backup/tasks/validate_config.yml

@@ -1,6 +1,6 @@
 ---
 
-- name: Fail if required Postgres settings not defined
+- name: Fail if required postgres-backup settings not defined
   ansible.builtin.fail:
     msg: >-
       You need to define a required configuration setting (`{{ item }}`).