fix(deps): update dependency starlight-blog to ^0.21.0

Author: renovate[bot]

package.json

@@ -40,7 +40,7 @@
     "sass": "^1.77.2",
     "sharp": "^0.33.2",
     "shiki": "^1.1.7",
-    "starlight-blog": "^0.15.0",
+    "starlight-blog": "^0.21.0",
     "starlight-links-validator": "^0.13.0"
   },
   "packageManager": "[email protected]",