Add net 5.0 as a target, with updated dependencies

[JohnCampionJr]

Hope I got this right :) All tests pass.

Closes #202

[JohnCampionJr]

So apparently GH CI doesn't yet have 5.0 preinstalled, so added it to Actions build flow

[Turnerj]

Strangely I can't get this to build in Visual Studio... I've got the .NET 5.0 SDK installed however something in the build process is stopping it from working.

NETSDK1005 Assets file 'C:\Users\James\Source\Repos\MongoFramework\src\MongoFramework\obj\project.assets.json' doesn't have a target for 'net5.0'. Ensure that restore has run and that you have included 'net5.0' in the TargetFrameworks for your project.

Cleaning the build doesn't work, neither manually deleting the obj and bin folders. Manually running dotnet restore doesn't fix this either for me. So while the code does compile and pass all tests on the CI, if I can't build it locally, I can't work on the library. 😟

This PR might be open for a little while until I can solve this problem on my end.

[JohnCampionJr]

That is odd. Compiles on both my Windows and Mac machines. Hope you find the problem.

[Turnerj]

Whatever issue I had before is gone... 🤷‍♂️ Thanks for your work on this - looks good!