build(deps): Bump third_party/cmark from 9d74662 to 3460cd8

Bumps [third_party/cmark](https://github.com/commonmark/cmark) from `9d74662` to `3460cd8`.
- [Release notes](https://github.com/commonmark/cmark/releases)
- [Commits](commonmark/cmark@9d74662...3460cd8)

---
updated-dependencies:
- dependency-name: third_party/cmark
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>