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GH-41349: [C#] Optimize DecimalUtility.GetBytes(SqlDecimal) on .NET 7+ (#42150)

### What changes are included in this PR?

Adds code to avoid an allocation when converting from SqlDecimal to Decimal128.
Adds a .NET 8 target to Apache.Arrow.csproj to enable the optimization.
Makes a small source change to build successfully with latest C# version.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

Closes #41349 
* GitHub Issue: #41349

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