chore(examples): migrates form-builder example to 3.0 (#9681)

### What?

Migrates the `form-builder` example to payload `3.0`.

`Updates`:

- Now has a next app directly along side payload.

- Removes `form-builder/next-app` & `form-builder/next-pages` example
front-ends and only uses new recommended approach (i.e admin panel &
front-end on the same port - `3000`)

examples/form-builder/src/components/Gutter/index.tsx

@@ -0,0 +1,30 @@
+import type { Ref } from 'react'
+
+import React, { forwardRef } from 'react'
+
+import classes from './index.module.scss'
+
+type Props = {
+  children: React.ReactNode
+  className?: string
+  left?: boolean
+  ref?: Ref<HTMLDivElement>
+  right?: boolean
+}
+
+export const Gutter: React.FC<Props> = forwardRef<HTMLDivElement, Props>((props, ref) => {
+  const { children, className, left = true, right = true } = props
+
+  return (
+    <div
+      className={[left && classes.gutterLeft, right && classes.gutterRight, className]
+        .filter(Boolean)
+        .join(' ')}
+      ref={ref}
+    >
+      {children}
+    </div>
+  )
+})
+
+Gutter.displayName = 'Gutter'