updates home page banner to point to v6

Author: sarah11918

src/content/docs/en/getting-started.mdx

@@ -7,7 +7,7 @@ editUrl: false
 next: false
 banner:
   content: |
-    Astro v5 is here! <a href="/en/guides/upgrade-to/v5/">Learn how to upgrade your site</a>
+    Astro v6 is here! <a href="/en/guides/upgrade-to/v6/">Learn how to upgrade your site</a>
 hero:
   title: Astro Docs
   tagline: Guides, resources, and API references to help you build with Astro.