could not validate a connection to Elasticsearch. No alive nodes found in your cluster

1. manually add “elasticsearch” to node <elasticsearch7_server_hostname> in vendor/magento/module-elasticsearch-7/etc/config.xml
2. disable elasticsearch6 and enable elasticsearch7 module

bin/magento module:enable {Magento_Elasticsearch,Magento_InventoryElasticsearch,Magento_Elasticsearch(version)}

bin/magento module:disable Magento_Elasticsearch(previous version)