chore(website): update dependency astro to v5.15.9 [security] (#7793)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

site/package-lock.json

@@ -18,7 +18,7 @@
   "dependencies": {
     "@astrojs/starlight": "0.36.0",
     "@bugsnag/js": "8.6.0",
-    "astro": "5.15.6",
+    "astro": "5.15.9",
     "markdown-magic": "2.6.1",
     "sharp": "0.34.4",
     "strip-ansi": "7.1.2"