[Build/CI] Upgrade xgrammar to 0.1.15

[russellb]

This update includes support for aarch64 among other fixes and improvements.

Closes #11886
Closes #13986

Implements part of #13894

Signed-off-by: Russell Bryant rbryant@redhat.com

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[WoosukKwon]

QQ: Does this upgrade increase the coverage of xgrammar? E.g., supporting regex, etc.

[russellb]

QQ: Does this upgrade increase the coverage of xgrammar? E.g., supporting regex, etc.

Yes, it lets us turn on regex support. #14590

It also fixes a bug that caused us to pin to 0.1.11 previously: #13338

[russellb]

This got merged as part of #14590