[csharp-netcore] Removed net5, added net7

[devhl-labs]

.net5 end of life was May 10, 2022.

https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core#servicing:~:text=out%20of%20support.-,End%20of%20life%20(EOL),-End%20of%20life

End of life (EOL)

End of life refers to the date when Microsoft no longer provides fixes, updates, or online technical assistance. End of life may also be referred to as 'end of support' (EOS).

As the end of life nears for a given .NET version, we recommend that you move to a newer .NET version, and reduce/remove your use of the given .NET version. After support ends, we recommend that you uninstall a given .NET version if you are no longer using it, or install the latest patch, and accelerate your plans to remove your use of that .NET version./

Your use of out-of-support .NET versions may put your applications, application data, and computing environment at risk. You are strongly recommended to not use out-of-support software.

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.
• 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 (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

[wing328]

tested passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/45414430 with new image.

will update doc and image after merging this PR.