chore(website): update dependency astro to v6

Author: renovate[bot]

site/package.json

@@ -18,7 +18,7 @@
   "dependencies": {
     "@astrojs/starlight": "0.37.7",
     "@bugsnag/js": "8.8.1",
-    "astro": "5.18.1",
+    "astro": "6.1.4",
     "markdown-magic": "2.6.1",
     "sharp": "0.34.5",
     "strip-ansi": "7.2.0"