Upgraded to .NET Core 5.0

[MarkMichaelis]

Description

• Upgraded the project to .NET 5.0
• Added option to prompt the user for API Key (rather than having to edit the code)
• Deleted test files/folder after uploading.
• Delete test files/folder during startup in case they were not deleted during a previous run.
• Added method PathExists to determine whether a path in Dropbox exists.

Checklist

General Contributing

• [x] Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• [ ] Non-code related change (markdown/git settings etc)
• [ ] SDK Code Change
• [x] Example/Test Code Change

Validation

• [x] Does this code build successfully?
• [x] Do all tests pass?
• [x] Does Stylecop pass?

[greg-db]

Thanks for putting this together! I'll ask the team to review this.

[MarkMichaelis]

I am updating the PR to include the OAuthPKCE project and then modify the respective Program.cs files to match as closely as possible, except for the relevant differences in approaches.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}