Bump katex and ngx-markdown

Bumps [katex](https://github.com/KaTeX/KaTeX) to 0.16.21 and updates ancestor dependency [ngx-markdown](https://github.com/jfcere/ngx-markdown). These dependencies need to be updated together.


Updates `katex` from 0.15.6 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@v0.15.6...v0.16.21)

Updates `ngx-markdown` from 13.1.0 to 19.0.0
- [Release notes](https://github.com/jfcere/ngx-markdown/releases)
- [Commits](jfcere/ngx-markdown@v13.1.0...v19.0.0)

---
updated-dependencies:
- dependency-name: katex
  dependency-type: indirect
- dependency-name: ngx-markdown
  dependency-type: direct:production
...

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