[Backport 8.1] Update docs for v8 (#1620)

Co-authored-by: Tomas Della Vedova <delvedor@users.noreply.github.com>

docs/advanced-config.asciidoc

@@ -45,9 +45,9 @@ is performed here, this means that if you want to swap the default HTTP client
 
 [source,js]
 ----
-const { Client, Connection } = require('@elastic/elasticsearch')
+const { Client, BaseConnection } = require('@elastic/elasticsearch')
 
-class MyConnection extends Connection {
+class MyConnection extends BaseConnection {
   request (params, callback) {
     // your code
   }