Bump tsdown from 0.18.3 to 0.21.1

Bumps [tsdown](https://github.com/rolldown/tsdown) from 0.18.3 to 0.21.1.
- [Release notes](https://github.com/rolldown/tsdown/releases)
- [Commits](rolldown/tsdown@v0.18.3...v0.21.1)

---
updated-dependencies:
- dependency-name: tsdown
  dependency-version: 0.21.1
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

packages/astro-integration/package.json

@@ -35,7 +35,7 @@
 	"devDependencies": {
 		"astro": "5.16.6",
 		"publint": "0.3.16",
-		"tsdown": "0.18.3",
+		"tsdown": "0.21.1",
 		"typescript": "5.9.3"
 	},
 	"peerDependencies": {