[csharp] Update RestSharp to 110.2.0

[OfficialRenny]

Bumped RestSharp to the latest stable version (110.2.0), + changes in ApiClient.cs to fix a couple errors caused by breaking changes in RestSharp.

• string ContentType(s) to RestSharp.ContentType ContentType(s).
• new RestClient([..]).UseSerializer() moved into the configureSerialization delegate constructor parameter.
• Moved the new RestResponse<T> Request initializer into the constructor as a RestRequest is now required.

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
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request. (@mandrean @frankyjuang @shibayan @Blackclaws @lucamazzanti)

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

Some CI jobs failed. Please take a look when you've time: https://github.com/OpenAPITools/openapi-generator/actions/runs/5589888558/jobs/10219041799?pr=16122

[OfficialRenny]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

Ah, thanks for letting me know! Should be sorted now

[wing328]

FYI. appveyor C# tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/47582481

[wing328]

appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/47603414

[johnwc]

The documentation was never updated to reference the new version requirements, it still says 108+ is required.
https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator/src/main/resources/csharp