fix(deps): update dependency tsdown to v0.20.3

Author: renovate[bot]

packages/@sanity/tsdown-config/package.json

@@ -56,7 +56,7 @@
     "vitest": "catalog:"
   },
   "peerDependencies": {
-    "tsdown": "0.20.x",
+    "tsdown": "0.20.3",
     "typescript": "5.8.x || 5.9.x"
   },
   "engines": {