Bump markdown from 5.0.0 to 6.0.0 in /web/_tool (#1376)

Bumps [markdown](https://github.com/dart-lang/markdown) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dart-lang/markdown/releases)
- [Changelog](https://github.com/dart-lang/markdown/blob/master/CHANGELOG.md)
- [Commits](https://github.com/dart-lang/markdown/compare/5.0.0...6.0.0)

---
updated-dependencies:
- dependency-name: markdown
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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web/_tool/pubspec.yaml

@@ -5,7 +5,7 @@ environment:
   sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
-  markdown: ^5.0.0
+  markdown: ^6.0.0
   path: ^1.8.0
 
 dev_dependencies: