fix(deps): update dependency @astrojs/starlight to ^0.21.1

Author: renovate[bot]

packages/documentation/package.json

@@ -9,7 +9,7 @@
     "astro": "astro"
   },
   "dependencies": {
-    "@astrojs/starlight": "^0.13.1",
+    "@astrojs/starlight": "^0.21.1",
     "@interledger/docs-design-system": "^0.3.0",
     "astro": "3.6.5",
     "astro-graphql-plugin": "^0.1.0",