node8: require nodejs >= 8.9.0, npm >= 6.4

Next version will be Etherpad 1.8. As planned in #3424, we are going to require
NodeJS >=8.9.0 and npm >= 6.4.

This commit implements that change and updates documentation and scripts.
Subsequent changes will get rid of old idioms, dating back to node < 0.7, that
still survive in the code.
Once migrated to NodeJS 8, we will be able to start working on migrating the
code base from callbacks to async/await, greatly simplifying legibility (see
#3540).

Closes #3557

src/node/server.js

@@ -32,12 +32,14 @@ log4js.replaceConsole();
  * early check for version compatibility before calling
  * any modules that require newer versions of NodeJS
  */
-NodeVersion.enforceMinNodeVersion('6.9.0');
+NodeVersion.enforceMinNodeVersion('8.9.0');
 
 /*
- * Since Etherpad 1.8.0, at least NodeJS 8.9.0 will be required
+ * As of Etherpad 1.8.0, we do not have any further Node version deprecation in
+ * place.
+ *
+ * NodeVersion.checkDeprecationStatus('10.13.0', '1.9.0');
  */
-NodeVersion.checkDeprecationStatus('8.9.0', '1.8.0');
 
 /*
  * start up stats counting system

src/package.json

@@ -73,8 +73,8 @@
     "wd": "1.11.1"
   },
   "engines": {
-    "node": ">=6.9.0",
-    "npm": ">=3.10.8"
+    "node": ">=8.9.0",
+    "npm": ">=6.4.1"
   },
   "repository": {
     "type": "git",