Bump xgrammar from 0.1.17 to 0.1.21

Bumps [xgrammar](https://github.com/mlc-ai/xgrammar) from 0.1.17 to 0.1.21.
- [Release notes](https://github.com/mlc-ai/xgrammar/releases)
- [Commits](mlc-ai/xgrammar@v0.1.17...v0.1.21)

---
updated-dependencies:
- dependency-name: xgrammar
  dependency-version: 0.1.21
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -234,7 +234,7 @@ websocket-client==1.8.0
 websockets==15.0.1
 wrapt==1.17.2
 xformers==0.0.29.post2
-xgrammar==0.1.17
+xgrammar==0.1.21
 xxhash==3.5.0
 yarl==1.20.0
 zipp==3.21.0