templates/docker-compose/ - update Borgmatics image

It's necessary to update to the `latest` image because the
`latest-msmtp` image is now unmaintained and doesn't support postgres
17.

However, this means we need to adjust things (the backup script and
the environment variable config) because we need to use the latest
notification mechanism, `apprise` instead of msmtp.

Tested, seems to be working.

templates/docker-compose/borgmatic.env.j2

@@ -1,2 +1,3 @@
 POSTGRES_PASSWORD={{ nextcloud_db_password }}
 BORG_PASSPHRASE={{ borg_passphrase }}
+SMTP_PASSWORD={{ smtp_password }}