fix(PostCSS): disable "Complex selectors" warning

We don't need an `:is()` polyfill, so the warning is just an
annoyance.

Also, remove explicit `autoprefixer` as it's included in
`postcss-preset-env`.

specifyweb/frontend/js_src/package.json

@@ -92,7 +92,6 @@
     "@types/react-modal": "^3.13.1",
     "@types/react-router-dom": "^5.3.3",
     "@types/underscore": "^1.10.24",
-    "autoprefixer": "10.4.5",
     "babel-jest": "^28.1.3",
     "babel-loader": "^8.2.5",
     "chalk": "^4.1.2",

specifyweb/frontend/js_src/postcss.config.js

@@ -1,7 +1,14 @@
 import postcssPresetEnv from 'postcss-preset-env';
 import tailwindcss from 'tailwindcss';
-import autoprefixer from 'autoprefixer';
 
 export default {
-  plugins: [postcssPresetEnv, tailwindcss, autoprefixer],
+  plugins: [
+    postcssPresetEnv({
+      features: {
+        // Disable warning about "Complex selectors"
+        'is-pseudo-class': false,
+      },
+    }),
+    tailwindcss,
+  ],
 };