Upgrade various grunt dependencies

grunt-concurrent:
* https://github.com/sindresorhus/grunt-concurrent/releases/tag/v3.0.0
* Breaking: Require Node.js 8+.

grunt-contrib-clean:
* https://github.com/gruntjs/grunt-contrib-clean/blob/v2.0.1/CHANGELOG
* Breaking: Require Node.js 12+.

grunt-contrib-pug:
* https://github.com/gruntjs/grunt-contrib-pug/blob/v3.0.0/CHANGELOG
* Breaking: Require Node.js 8+.
* Upgrade from Pug 2 to Pug 3.
  https://pugjs.org/api/migration-v3.html
  - Change `minify` (unused).
  - Change `plugins` (unused).
  - Require Node.js 12+.

grunt-contrib-uglify:
* https://github.com/gruntjs/grunt-contrib-uglify/blob/v5.2.2/CHANGELOG
* Breaking: Require Node.js 12+.
* Upgrade uglify-js from 3.4.0 to ~3.15.2.

grunt-html:
* https://github.com/validator/grunt-html/releases
  https://github.com/validator/grunt-html/commits/v10.0.0/
  https://github.com/validator/grunt-html/commits/v11.0.0/
  https://github.com/validator/grunt-html/releases/tag/v15.0.0
* grunt-html 10, 11:
  - vnu-jar 19.x
* grunt-html 12, 13, 14:
  - vnu-jar 20.x.
  - Require Node.js 10+.
* grunt-html 15:
  - vnu-jar 21.x.
  - Require Node.js 12+
  - Change JSON reporter (unused?).
* grunt-html 16, 17:
  - vnu-jar 23.x.
  - require Node.js 14+

Gruntfile.js

@@ -173,6 +173,11 @@ module.exports = function(grunt) {
     },
 
     htmllint: {
+      options: {
+        ignore: [
+          /Trailing slash on void elements has no effect/
+        ]
+      },
       src: 'build/**/*.html'
     },
 

package.json

@@ -9,8 +9,7 @@
   "repository": "gruntjs/gruntjs.com",
   "license": "MIT",
   "engines": {
-    "node": ">=6",
-    "npm": ">=3"
+    "node": ">=18"
   },
   "scripts": {
     "postinstall": "grunt build",
@@ -28,13 +27,13 @@
     "ent": "~2.2.0",
     "errorhandler": "~1.5.0",
     "express": "~4.16.3",
-    "grunt": "~1.0.3",
+    "grunt": "~1.6.1",
     "grunt-autoprefixer": "~3.0.4",
     "grunt-contrib-clean": "~1.1.0",
     "grunt-contrib-copy": "~1.0.0",
-    "grunt-contrib-less": "~2.0.0",
-    "grunt-contrib-pug": "^2.0.0",
-    "grunt-contrib-uglify": "~4.0.0",
+    "grunt-contrib-less": "~2.1.0",
+    "grunt-contrib-pug": "~3.0.0",
+    "grunt-contrib-uglify": "~5.2.2",
     "grunt-docs": "https://github.com/gruntjs/grunt-docs.git#d224f17ae21fe606b5b7eca73155a31f4e341930",
     "grunt-sitemap": "~1.2.1",
     "highlight.js": "~9.12.0",
@@ -43,14 +42,14 @@
     "marked": "https://github.com/vladikoff/marked/tarball/032a72231503b63e15e76b990a914becf664b0d7",
     "method-override": "~3.0.0",
     "node-schedule": "~1.3.0",
-    "pug": "~2.0.3",
+    "pug": "~3.0.3",
     "request": "~2.88.0"
   },
   "devDependencies": {
-    "grunt-concurrent": "~2.3.1",
+    "grunt-concurrent": "~3.0.0",
     "grunt-contrib-jshint": "~1.1.0",
     "grunt-contrib-watch": "~1.1.0",
-    "grunt-html": "~9.3.0",
+    "grunt-html": "~17.0.1",
     "grunt-nodemon": "~0.4.2",
     "grunt-open": "^0.2.4",
     "grunt-puglint": "~1.0.0"