perf: bump html-rspack-plugin 6.0.0-beta.8

[chenjiahan]

Summary

html-rspack-plugin 6.0.0-beta.8 includes some small performance improvements.

See: https://github.com/rspack-contrib/html-rspack-plugin/commits/main/

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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