Upgrade versions in package.json

Author: philwebb

package.json

@@ -5,31 +5,31 @@
   "repository": "https://github.com/spring-io/spring-doc-resources",
   "license": "Apache-2.0",
   "devDependencies": {
-    "autoprefixer": "^9.8.6",
+    "autoprefixer": "^10.4.2",
     "browser-pack-flat": "^3.4.2",
     "browserify": "^17.0.0",
     "cssnano": "^5.0.16",
     "eslint": "^7.20.0",
     "eslint-config-prettier": "^8.1.0",
     "eslint-config-standard": "^16.0.2",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-buffer": "^0.0.2",
     "gulp-concat": "^2.6.1",
     "gulp-connect": "^5.7.0",
     "gulp-eslint7": "^0.3.0",
     "gulp-header": "^2.0.9",
     "gulp-postcss": "^9.0.0",
-    "gulp-sourcemaps": "^2.6.5",
+    "gulp-sourcemaps": "^3.0.0",
     "gulp-stylelint": "^13.0.0",
     "gulp-tap": "^2.0.0",
     "gulp-terser": "^2.0.1",
     "gulplog": "^1.0.0",
-    "highlight.js": "^10.6.0",
+    "highlight.js": "^11.4.0",
     "lodash": "^4.17.21",
     "postcss": "^8.2.6",
     "postcss-import": "^14.0.0",
-    "prettier": "2.2.1",
-    "stylelint": "^13.11.0",
+    "prettier": "2.5.1",
+    "stylelint": "^13.0.0",
     "stylelint-config-standard": "^20.0.0"
   },
   "scripts": {