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GH-25163: [C#] Support half-float arrays. (#34618)

### Rationale for this change

.NET 5 introduced the [`System.Half`](https://devblogs.microsoft.com/dotnet/introducing-the-half-type/) type, which represents 16-bit floats. This PR adds support for them in Apache Arrow.

### What changes are included in this PR?

I multi-targeted the `Apache.Arrow` project to .NET 6 (because .NET 5 is unsupported) and added a `HalfFloatArray` type with a very similar implementation as the other floating-point array types.

I also updated the README.

### Are these changes tested?

Yes. I also refactored the array tests to reduce duplication among the various numeric types.

### Are there any user-facing changes?

Yes.
* Closes: #25163

Lead-authored-by: Theodore Tsirpanis <teo@tsirpanis.gr>
Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>