Drop support for older frameworks/Add support for newer frameworks

[tsahi]

Is your feature request related to a problem? Please describe.
Following the discussion at #38, we still support old frameworks that went out of support. We should remove support for these frameworks, which will let us remove dependency on older NuGet Packages.
on the other hand, adding direct support for newer frameworks will let us add support for ADO.NET providers we don't currently support on all frameworks.

Describe the solution you'd like
We currently support the following frameworks directly:

• net45
• net46
• net47
• netstandard2.0 (except for SqlCe)
• netcoreapp2.0 (except for OleDb and SqlCe)
• netcoreapp3.0 (except for SqlCe)

As a result, we support OleDb from netcoreapp3.0, although it's available for netcoreapp2.1.

I suggest we:

1. drop support for net45, and replace it with support for net452. This will let us support System.Data.SqlClient NuGet package for this version. Support for Microsoft.Data.SqlClient is available from net46.
2. drop support for netcoreapp2.0, and replace it with support for netcoreapp2.1. This will let us support System.Data.OleDb NuGet package from this version, instead of from version 3.0.
3. (optional) drop support for netcoreapp3.0, and replace it with support for netcoreapp3.1.
4. (optional) add support for netstandard1.2 on the SqlServer package (Split the block into separate packages for each database #39). We can support this package on this version using System.Data.SqlClient, and Microsoft.Data.SqlClient starting from netstandard2.0. The question is will this require the common Data package to also support this version.

Additional context
These are the End Of Support dates for .NET Core versions, from the Microsoft web site:

Version	End of Support Date
Core 2.0	October 1, 2018
Core 2.1	August 21, 2021
Core 2.2	December 23, 2019
Core 3.0	March 3, 2020
Core 3.1	December 3, 2022
5.0	around February 2022

As we can see, support for .NET Core 2.0 and 2.2 is long overdue, and .NET Core 3.0 also recently went out of support.

Support for .NET Framework 4.5 ended January 12, 2016, and is replaced with version 4.5.2.

[tsahi]

@Chavoshi @turabek @mjrousos

[tsahi]

see also #41

[tsahi]

supporting netstandare1.2 on SqlServer requires supporting it on Data too, but Data depends on System.Data.Odbc and System.Configuration.ConfigurationManager, which are supported on netstandard2.0 and higher. So this would be impossible.