Bump katex and ngx-markdown in /webapp/frontend

Bumps [katex](https://github.com/KaTeX/KaTeX) to 0.16.28 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 0.16.28
- [Release notes](https://github.com/KaTeX/KaTeX/releases)
- [Changelog](https://github.com/KaTeX/KaTeX/blob/main/CHANGELOG.md)
- [Commits](https://github.com/KaTeX/KaTeX/compare/v0.15.6...v0.16.28)

Updates `ngx-markdown` from 13.1.0 to 21.1.0
- [Release notes](https://github.com/jfcere/ngx-markdown/releases)
- [Commits](https://github.com/jfcere/ngx-markdown/compare/v13.1.0...v21.1.0)

---
updated-dependencies:
- dependency-name: katex
  dependency-version: 0.16.28
  dependency-type: indirect
- dependency-name: ngx-markdown
  dependency-version: 21.1.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

webapp/frontend/package.json

@@ -38,7 +38,7 @@
         "lodash": "4.17.23",
         "moment": "^2.29.4",
         "ng-apexcharts": "^1.7.4",
-        "ngx-markdown": "^13.1.0",
+        "ngx-markdown": "^21.1.0",
         "perfect-scrollbar": "^1.5.5",
         "quill": "^1.3.7",
         "rrule": "^2.7.1",