[Backport 7.x] Add warning log about nodejs version support (#1350)

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

index.js

@@ -41,6 +41,24 @@ const kEventEmitter = Symbol('elasticsearchjs-event-emitter')
 
 const ESAPI = require('./api')
 
+/* istanbul ignore next */
+if (nodeMajor < 10) {
+  process.emitWarning('You are using a version of Node.js that is currently in EOL. ' +
+                      'The support for this version will be dropped in 7.12. ' +
+                      'Please refer to https://ela.st/nodejs-support for additional information.',
+  'DeprecationWarning'
+  )
+}
+
+/* istanbul ignore next */
+if (nodeMajor >= 10 && nodeMajor < 12) {
+  process.emitWarning('You are using a version of Node.js that will reach EOL in April 2021. ' +
+                      'The support for this version will be dropped in 7.13. ' +
+                      'Please refer to https://ela.st/nodejs-support for additional information.',
+  'DeprecationWarning'
+  )
+}
+
 class Client extends ESAPI {
   constructor (opts = {}) {
     super({ ConfigurationError })