[core] Support WAHA_API_KEY as well as env variable

gpt often suggests this env variable, IDK why

.env.example

@@ -23,7 +23,7 @@ WAHA_BASE_URL=http://localhost:3000
 #
 # https://waha.devlike.pro/docs/how-to/security/
 #
-WHATSAPP_API_KEY=admin
+WAHA_API_KEY=admin
 WAHA_DASHBOARD_USERNAME=admin
 WAHA_DASHBOARD_PASSWORD=admin
 #WHATSAPP_SWAGGER_USERNAME=admin