fix: always use pgxpool config if MaxOpenConns isn't set (#8328)

# Which Problems Are Solved

- `pgxpool -> pgx` dependency throws "MaxSize must be >= 1" on init if
`postgres.MaxOpenConns` isn't set in the ZItadel config

# How the Problems Are Solved

Only override the `MaxConns` with the Zitadel configured `MaxOpenConns`
if greater than 0 (default value). The default `MaxConns` [is derived by
`pgxpool`](https://github.com/jackc/pgx/blob/ea9610f6722e0c6c6073660f14ded3c9683a593e/pgxpool/pool.go#L309-L324)
itself in a sensible way, but somewhat undocumented: checks for explicit
config in connection url or config and falls back on max(num_cpus, 4).

# Additional Changes

Applied same check in cockroach config 

# Additional Context

This is likely a regression from the changes in
https://github.com/zitadel/zitadel/pull/8325

internal/database/cockroach/crdb.go

@@ -81,7 +81,11 @@ func (c *Config) Connect(useAdmin bool, pusherRatio, spoolerRatio float64, purpo
 	if err != nil {
 		return nil, err
 	}
-	config.MaxConns = int32(connConfig.MaxOpenConns)
+
+	if connConfig.MaxOpenConns != 0 {
+		config.MaxConns = int32(connConfig.MaxOpenConns)
+	}
+
 	config.MaxConnLifetime = c.MaxConnLifetime
 	config.MaxConnIdleTime = c.MaxConnIdleTime
 

internal/database/postgres/pg.go

@@ -82,7 +82,11 @@ func (c *Config) Connect(useAdmin bool, pusherRatio, spoolerRatio float64, purpo
 	if err != nil {
 		return nil, err
 	}
-	config.MaxConns = int32(connConfig.MaxOpenConns)
+
+	if connConfig.MaxOpenConns != 0 {
+		config.MaxConns = int32(connConfig.MaxOpenConns)
+	}
+
 	config.MaxConnLifetime = c.MaxConnLifetime
 	config.MaxConnIdleTime = c.MaxConnIdleTime