Bump katex from 0.16.18 to 0.16.21

Bumps [katex](https://github.com/KaTeX/KaTeX) from 0.16.18 to 0.16.21.
- [Release notes](https://github.com/KaTeX/KaTeX/releases)
- [Changelog](https://github.com/KaTeX/KaTeX/blob/main/CHANGELOG.md)
- [Commits](KaTeX/KaTeX@v0.16.18...v0.16.21)

---
updated-dependencies:
- dependency-name: katex
  dependency-type: indirect
...

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Author: dependabot[bot]