fix: update migration banner

Author: mehalter

src/content/docs/index.mdx

@@ -6,7 +6,7 @@ head:
     content: Getting Started
 banner:
   content: |
-    AstroNvim v4 is officially released! Upgrade today with the <a href="https://docs.astronvim.com/configuration/v4_migration/">v4 migration guide</a>!
+    AstroNvim v5 is officially released! Upgrade today with the <a href="https://docs.astronvim.com/configuration/v5_migration/">v5 migration guide</a>!
 ---
 
 import { Tabs, TabItem, FileTree, Steps } from "@astrojs/starlight/components";