[csharp] Obsolete attribute on deprecated operations and properties

[NathanBaulch]

Apply the obsolete attribute to deprecated methods and properties.

@mandrean @frankyjuang @shibayan @Blackclaws @lucamazzanti

PR checklist

• Read the contribution guidelines.

• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.

• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

• File the PR against the correct branch: master, 5.3.x, 6.0.x

• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

Appveyor CI failure not related to this change.

[wing328]

LGTM. Thanks for the PR.