firefly-iii: Connect to external PostgreSQL

Since the new database server outside the Kubernetes cluster, created
for Authelia, was seeded from a backup of the in-cluster server, it
already contained the data from Firefly-III as well.  Thus, we can
switch Firefly-III to using it, too.

The documentation for Firefly-III does not mention anything about how
to configure it to use certificate-based authentication for PostgreSQL,
as is required by the new server.  Fortunately, it ultimately uses
_libpq_, so the standard `PG...` environment variables work fine.  We
just need a certificate issued by the _postgresql-ca_ ClusterIssuer and
the _DCH Root CA_ certificate mounted in the Firefly-III container.

firefly-iii/firefly-iii.yaml

@@ -73,8 +73,6 @@ spec:
         env:
         - name: APP_KEY_FILE
           value: /run/secrets/firefly-iii/app.key
-        - name: DB_PASSWORD_FILE
-          value: /run/secrets/firefly-iii/db.password
         - name: STATIC_CRON_TOKEN_FILE
           value: /run/secrets/firefly-iii/cron.token
         ports: