fix(cmd): clarify notification config handling (#9459)

# Which Problems Are Solved

If configuration `notifications.LegacyEnabled` is set to false when
using cockroachdb as a database Zitadel start does not work and prints
the following error: `level=fatal msg="unable to start zitadel"
caller="github.com/zitadel/zitadel/cmd/start/start_from_init.go:44"
error="can't scan into dest[0]: cannot scan NULL into *string"`

# How the Problems Are Solved

The combination of the setting and cockraochdb are checked and a better
error is provided to the user.

# Additional Context

- introduced with https://github.com/zitadel/zitadel/pull/9321

cmd/start/start.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/tls"
 	_ "embed"
+	"errors"
 	"fmt"
 	"math"
 	"net/http"
@@ -268,6 +269,9 @@ func startZitadel(ctx context.Context, config *Config, masterKey string, server
 	actionsLogstoreSvc := logstore.New(queries, actionsExecutionDBEmitter, actionsExecutionStdoutEmitter)
 	actions.SetLogstoreService(actionsLogstoreSvc)
 
+	if !config.Notifications.LegacyEnabled && dbClient.Type() == "cockroach" {
+		return errors.New("notifications must be set to LegacyEnabled=true when using CockroachDB")
+	}
 	q, err := queue.NewQueue(&queue.Config{
 		Client: dbClient,
 	})