Collaborators: Regenerate nuget.org API key

Every year the nuget.org API keys expire and must be regenerated. A reminder email is sent out to the registered owners on nuget.org.

• Log in to https://www.nuget.org/account/ApiKeys with an account that is admin of UnitsNet organization
• Create or regenerate key with name AppVeyor for UnitsNet project
  • Expires in: 365 days
  • Package owner: UnitsNet
  • Select scopes: Push > Push new packages and package versions
  • Select packages > Glob pattern: * to include all UnitsNet packages
• Copy the API key
• Encrypt the API key by pasting it into https://ci.appveyor.com/tools/encrypt (log in with account that is admin of UnitsNet project)
• Update appveyor.yml with the ENCRYPTED API key

deploy:
- provider: NuGet
  api_key:
    secure: Bc59XzyBNSWbOvxFp8Fyv2v5TVs5NY/rFnz/BA1lA7A35qrS3Ngrc8YFe/zuSXfF
  on:
    branch: master

- provider: NuGet
  api_key:
    secure: Bc59XzyBNSWbOvxFp8Fyv2v5TVs5NY/rFnz/BA1lA7A35qrS3Ngrc8YFe/zuSXfF
  on:
    branch: release/*

• Commit and push to master, or go via pull request if you want some extra eyes on it
• Bump nuget versions to test the new API key
  • On master branch, run https://github.com/angularsen/UnitsNet/blob/master/Build/bump-version-UnitsNet-patch.bat
  • Push the bumped version commits
• Verify that within 20 minutes, the new UnitsNet package version is out on nuget.org
  • Monitor or troubleshoot the build at: https://ci.appveyor.com/project/angularsen/unitsnet/
  • You will typically receive an email when the new package version is made available