fix: correct expToWebhookConfig cache locking (#10014)

There is a write under a read lock, causing the master to crash in integration tests with `fatal error: concurrent map writes`.

master/internal/webhooks/postgres_webhook.go

@@ -148,8 +148,8 @@ func (l *WebhookManager) getWebhookConfig(ctx context.Context, expID *int) (*exp
 		return nil, nil
 	}
 
-	l.mu.RLock()
-	defer l.mu.RUnlock()
+	l.mu.Lock()
+	defer l.mu.Unlock()
 
 	if config, ok := l.expToWebhookConfig[*expID]; ok {
 		return config, nil