Support for .NET 7

With version 2.2.0 we now support building using the .NET 7 SDK. Thanks to @ErikEJ for his contribution.

Please note that with this release we've also dropped support for .NET 5 as it is no longer supported by Microsoft. If you're still using .NET 5 we recommend you keep using your current version of MSBuild.Sdk.SqlProj until you're ready to upgrade to .NET 6 or 7.