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GH-31579 [C#]: Remove out-of-support versions of .NET and update C# README (#39165)

### What changes are included in this PR?

The project file no longer builds targets for netstandard1.3 or netcoreapp3.1.
The C# README.md is a little more up-to-date.

### Are these changes tested?

N/A

* Closes: #31579

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>