Replace custom/matrix-postgres-backup role with galaxy/com.devture.ansible.role.postgres_backup

This role is usable on its own and it's not tied to Matrix, so
extracting it out into an independent role that we install via
ansible-galaxy makes sense.

This also fixes the confusion from the other day, where
`matrix_postgres_*` had to be renamed to `devture_postgres_*`
(unless it was about `matrix_postgres_backup_*`).
We now can safely say that ALL `matrix_postgres_*` variables need to be
renamed.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2305

playbooks/matrix.yml

@@ -97,8 +97,11 @@
     - custom/matrix-nginx-proxy
     - custom/matrix-coturn
     - custom/matrix-aux
-    - custom/matrix-postgres-backup
+
+    - role: galaxy/com.devture.ansible.role.postgres_backup
+
     - custom/matrix-backup-borg
+
     - custom/matrix-user-creator
     - custom/matrix-common-after