Chore: Postcss config clean up (#4729)

Because:
- I've been looking at other Postcss configurations for similar apps, we
don't need some of our plugins and can probably simplify.

This commit
- Removes postcss-preset-env
- Removes postcss-cli - we aren't using this anywhere
- Removes postcss-flexbugs-fixes - we might need this again, but I want
to see if any issues are reported.

package.json

@@ -30,10 +30,7 @@
     "flatpickr": "^4.6.13",
     "mermaid": "^10.9.1",
     "postcss": "^8.4.40",
-    "postcss-cli": "^11.0.0",
-    "postcss-flexbugs-fixes": "^5.0.2",
     "postcss-import": "^16.1.0",
-    "postcss-preset-env": "^10.0.0",
     "prismjs": "^1.29.0",
     "regenerator-runtime": "^0.14.1",
     "sortablejs": "^1.15.0",

postcss.config.js

@@ -1,14 +1,8 @@
 module.exports = {
   plugins: [
     require('postcss-import'),
-    require('postcss-flexbugs-fixes'),
     require("tailwindcss/nesting"),
     require("tailwindcss"),
-    require('postcss-preset-env')({
-      autoprefixer: {
-        flexbox: 'no-2009'
-      },
-      stage: 3
-    }),
+    require("autoprefixer"),
   ]
 }