Add a framework version check for .NET Framework 4.5+

Figure out and implement a plan for dealing with the supported framework versions, especially now that Excel-DNA itself does not support .NET Framework 4.0 anymore.
The framework version is different to the runtime version, so I think we need a separate attribute in the .dna file:

```

```

We'll keep RuntimeVersion in place for now, although the only supported value is currently "v4.0", imagining a future where "v5" makes sense.