Cleanup projectfiles and dependencies

[marcovisserFurore]

Description

• dependency updates of the following packages:
  • Microsoft.NET.Test.Sdk: 16.11.0
  • MSTest.TestAdapter: 2.2.7
  • MSTest.TestFramework: 2.2.7
  • FluentAssertions: 6.1.0
  • BenchmarkDotNet: 0.13.1
• Adding README.md to the NuGet package. See also https://docs.microsoft.com/en-us/nuget/nuget-org/package-readme-on-nuget-org
• removing files that are not used anymore, like target files
• move TargetFrameworks in firely-net-sdk.props
• target framework net6.0 instead of net5.0

Related issues

See also PR: FirelyTeam/firely-net-common#162

FirelyTeam Checklist

• Update the title of the PR to be succinct and less than 50 characters
• Mark the PR with the label breaking change when this PR introduces breaking changes