avro

Bump NUnit from 3.13.1 to 3.13.2 in /lang/csharp (#1207)

Bumps [NUnit](https://github.com/nunit/nunit) from 3.13.1 to 3.13.2.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/master/CHANGES.md)
- [Commits](https://github.com/nunit/nunit/compare/v3.13.1...v3.13.2)

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