avro

Bump System.CodeDom from 4.4.0 to 6.0.0 in /lang/csharp (#1401)

Bumps [System.CodeDom](https://github.com/dotnet/runtime) from 4.4.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](https://github.com/dotnet/runtime/commits/v6.0.0)

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

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