Replace grunt-postcss with postcss-cli (#20140)

Refs #19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

package.json

@@ -13,12 +13,16 @@
   ],
   "homepage": "http://getbootstrap.com",
   "author": "The Bootstrap Authors (https://github.com/twbs/bootstrap/graphs/contributors)",
-  "contributors": ["Twitter, Inc."],
+  "contributors": [
+    "Twitter, Inc."
+  ],
   "scripts": {
     "change-version": "node grunt/change-version.js",
     "shrinkwrap": "npm shrinkwrap --dev && mv ./npm-shrinkwrap.json ./grunt/npm-shrinkwrap.json",
     "eslint": "eslint --config js/.eslintrc.json js/src",
     "jscs": "jscs --config=js/.jscsrc js/src js/tests/unit docs/assets/js/src docs/assets/js/ie*.js grunt Gruntfile.js",
+    "postcss": "postcss --config grunt/postcss.js --replace dist/css/*.css",
+    "postcss-docs": "postcss --config grunt/postcss.js --no-map --replace docs/assets/css/docs.min.css && postcss --config grunt/postcss.js --no-map --replace docs/examples/**/*.css",
     "test": "npm run eslint && npm run jscs && grunt test"
   },
   "style": "dist/css/bootstrap.css",
@@ -56,7 +60,6 @@
     "grunt-exec": "^0.4.6",
     "grunt-html": "^7.0.0",
     "grunt-jekyll": "^0.4.2",
-    "grunt-postcss": "^0.8.0",
     "grunt-sass": "^1.0.0",
     "grunt-saucelabs": "^8.6.1",
     "grunt-scss-lint": "^0.3.8",
@@ -64,7 +67,7 @@
     "is-travis": "^1.0.0",
     "jscs": "^3.0.4",
     "load-grunt-tasks": "^3.4.0",
-    "mq4-hover-shim": "^0.3.0",
+    "postcss-cli": "^2.5.2",
     "postcss-flexbugs-fixes": "^2.0.0",
     "shelljs": "^0.7.0",
     "time-grunt": "^1.2.1"