Bump xgrammar from 0.1.20 to 0.1.21

Bumps [xgrammar](https://github.com/mlc-ai/xgrammar) from 0.1.20 to 0.1.21.
- [Release notes](https://github.com/mlc-ai/xgrammar/releases)
- [Commits](mlc-ai/xgrammar@v0.1.20...v0.1.21)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -282,7 +282,7 @@ websockets==15.0.1
 widgetsnbextension==4.0.9
 wrapt==1.17.2
 xformers==0.0.28.post3
-xgrammar==0.1.20
+xgrammar==0.1.21
 yarl==1.20.1
 zipp==3.23.0
 zstandard==0.23.0