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GH-39916: [C#] Restore support for .NET 4.6.2 (#40008)

### What changes are included in this PR?

Project targets have been added for net462 which is still in support. A few tests have been modified to allow them to build against that target.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

There are new build artifacts for Apache.Arrow.dll and Apache.Arrow.Compression.dll.

* Closes: #39916

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