Update integration tests: PostCSS re-emits the UTF-8 BOM character (#20372)

This PR updates the integration tests that started failing because of a
recent PostCSS update.

PostCSS v8.5.24 started re-emitting the BOM character in the output,
see: https://github.com/postcss/postcss/releases/tag/8.5.24 therefore
the tests started failing.

This PR just re-adds that back.

It might not be clear from the diff on GitHub, but this is the change: 
<img width="878" height="84" alt="image"
src="https://github.com/user-attachments/assets/5e2fbb6e-2ee4-46c5-9a2c-9c3a1a806bb0"
/>


## Test plan

1. All tests pass again [ci-all]

integrations/upgrade/index.test.ts

@@ -2817,7 +2817,7 @@ test(
       <div class="ring-3"></div>
 
       --- ./src/input.css ---
-      @import 'tailwindcss';
+      @import 'tailwindcss';
 
       /*
         The default border color has changed to \`currentcolor\` in Tailwind CSS v4,