add support for .NET 5

[mastersign]

• Add net5.0 to the TFMs
• Introduce conditional dependency references
  • For .NET Framework 4 use Npgsql 4.1.6 and include reference to System.Transactions
  • For .NET Standard 2 and .NET 5 use Npgsql 5.0.0

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Rebus is MIT-licensed. The code submitted in this pull request needs to carry the MIT license too. By leaving this text in, I hereby acknowledge that the code submitted in the pull request has the MIT license and can be merged with the Rebus codebase.

[CLAassistant]

All committers have signed the CLA.

[mastersign]

It looks to me in the test log of appveyor, that the tests are only run against .NET Framework 4. If I run dotnet test locally (with the .NET 5.0 SDK CLI) it picks up on all three TFMs of the test project .NET Framework 4, .NET Core 3.1 and .NET 5.0 and all tests succeeded. But in appveyor the autmatically generated test command seems to run only the first TFM.

[mookid8000]

Excellent, thanks! 🙂 it's out as Rebus.PostgreSql 7.2.0 on NuGet.org in a few minutes