.NET 8 SDK Upgrade

[pedropaulovc]

This PR updates repo to build with the latest .NET 8 SDK and NuGet packages. Fixes #55.

[jviau]

Major version-reving Microsoft.Extensions.* dependencies to 8.0.0 is a breaking change for customers (as it can introduce package conflicts or be an environment restriction for them).

I see this is a dependency of your other PR though (at least the 6.0 packages) though. I think what will need to happen is one of two things:

1. Multi-target all these packages, adding at least net6.0 TFM and then only in those versions do we add the new IHostApplicationBuilder APIs
   OR
2. Update to the minimal version that includes the IHostApplicationBuilder (net6.0?) and then will also need to major version rev these packages.

[jviau]

nit: add newline at end of file.

[jviau]

Does this need to be upgraded? I prefer to leave dependencies of a library at a minimum for what the code base needs directly. End users can then decide to pin transitive dependencies at a higher version if they need.

This lets this library capture the widest customer base possible: if a project for some reason cannot update a transitive dependency, this will not force them to.

[jviau]

I'd prefer not to force all users of this library onto 8.0.0 versions of Microsoft.Extensions.*. Upgrading major versions of a transitive dependency is a breaking change to users.