Add missing wasp() plugin to vite.config.ts examples in Tailwind migration guide (#3840)

The example in the 'Option B: Upgrade to Tailwind CSS v4' section of the migration guide was missing the required wasp() plugin import and call. This fix updates both the current docs and the versioned 0.21 docs to match the correct example in the Tailwind CSS guide.

Author: cprecioso

web/docs/migration-guides/migrate-from-0-20-to-0-21.md

@@ -309,12 +309,14 @@ Wasp no longer manages Tailwind CSS internally, so there are a few changes regar
 1. Add the `@tailwindcss/vite` plugin to your `vite.config.{js,ts}` file.
 
     ```ts title="vite.config.ts"
+    import { wasp } from 'wasp/client/vite';
     // highlight-next-line
     import tailwindcss from '@tailwindcss/vite';
     import { defineConfig } from 'vite';
 
     export default defineConfig({
       plugins: [
+        wasp(),
         // highlight-next-line
         tailwindcss(),
       ],

web/versioned_docs/version-0.21/migration-guides/migrate-from-0-20-to-0-21.md

@@ -309,12 +309,14 @@ Wasp no longer manages Tailwind CSS internally, so there are a few changes regar
 1. Add the `@tailwindcss/vite` plugin to your `vite.config.{js,ts}` file.
 
     ```ts title="vite.config.ts"
+    import { wasp } from 'wasp/client/vite';
     // highlight-next-line
     import tailwindcss from '@tailwindcss/vite';
     import { defineConfig } from 'vite';
 
     export default defineConfig({
       plugins: [
+        wasp(),
         // highlight-next-line
         tailwindcss(),
       ],