Consider removing out of support TFMs

[ViktorHofer]

From the README.md:

.NET Standard support: It must run on the full .NET Framework and all platforms supported by .NET Core.

Most libraries already target .NET Standard 2.0 or a supported platform that support .NET Standard 2.0 like

• .NET Core 2.0 and above
• .NET Framework 4.6.1 and above
• UAP10.0.16299

I would suggest to consider dropping support for .NET Standard 1.3 and older .NET Framework versions in favor of a broader API set and a simplified dependency graph. Other open source repositories are dropping support for older targets as well, i.e. xunit v3. I would be willing to send a PR but wanted to open an issue to discuss this first.

[bgrainger]

Duplicate of #636; let's move discussion there.

[bgrainger]

I think I see a trend here... any particular reason why this is an important issue for MySqlConnector?

[bgrainger]

Duplicate of #636