Change default target framework to netstandard2.1

[zijchen]

• Default to netstandard2.1 instead of the net46 from SqlTasks.target, also allows it to be overridable with anything like net6.0 or net8.0 - fixes SQLProj Targets Wrong Framework Despite Explicit net6.0 Specification #330
• Import Microsoft.NET.Sdk.targets which adds more features as supported by .NET such as transitive project dependencies - fixes SDK based project does not cover the transitive dependencies on the project reference #278

[chlafreniere]

What will our story be for .net framework?

[zijchen]

What will our story be for .net framework?

Since sqlproj doesn't really care about .NET version, you can set the target framework to net46 if you want. The test confirms that.

If you're asking about using the .NET Framework version of msbuild, nothing is changed in that path since we need the targets from the .NET SDK.