Remove `netstandard1.3` support

[bgrainger]

Now that MySqlConnector is targeting net6.0, it seems very unnecessary to also keep targeting netstandard1.3. There are a lot of #if blocks in the code purely for that target framework, and removing it would simplify a lot of code.

This also aligns with (part of) Microsoft's decision for their packages: dotnet/announcements#190

MySqlConnector 1.3.x is quite stable, and could continue to be used by netstandard1.3 consumers.

(Split from #636.)