Can't use IDbColumnSchemaGenerator from .NET 4.7.2

[bgrainger]

IDbColumnSchemaGenerator is available since .NET 4.7.1. However, if you install MySqlConnector into a .NET 4.7.2 project, it references the net461 library, which doesn't use that interface, rather than the netstandard2.0 library (which might be more appropriate).

We probably need to fix this by targeting net471 (which introduced the interface) for .NET Framework clients.

We should also check if any other new APIs are supported by net471 (and greater).

Finally, the #if check is wrong and blocks the API from netcoreapp2.1. Most likely, all #if checks should be rewritten to be negative, so that APIs are enabled by default.

[bgrainger]

Fixed in 0.48.1.