Upgraded from .NET 4.5 to .NET 4.8 - .NET 4.5 is no longer supported

[marknoble]

When I opened this, I saw that it was based on .NET 4.5 which is no longer supported, so I upgraded the project in this fork and checked in the upgraded version after confirming that it builds without errors.

This should save a few steps for anyone working with these API samples in .NET going forward.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[marknoble]

@AndyDiamondstein could you please review? I was following up on old pull requests, and saw I failed the CLA check. Figured that out and got it to pass, but also noticed that most PR's are closed when faced with same. Seems that the project may benefit from the addition of a CONTRIBUTING.md file that directs users to resources on how to sign a CLA.