Unity complains about netstandard2.1 on package import

Everything seems to be working in the C# SDK but I get this error when I import the SDK:
```
SpacetimeDB.BSATN.Runtime references netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51. A roslyn analyzer should reference netstandard version 2.0
```

We should investigate if we can fix this.