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GH-41375: [C#] Move to .NET 8.0 (#41376)

### What changes are included in this PR?

Changes to workflow infrastructure and projects to install and target net8.0 instead of net7.0.

### Are these changes tested?

Yes

### Are there any user-facing changes?

Users will need to install .NET 8 to run tests and examples. No impact on product code.

Closes #41375
* GitHub Issue: #41375

Lead-authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>