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GH-34661: [CI][C#] Update Ubuntu C# jobs to use image with .NET 7.0 (#34662)

### Rationale for this change
Nightly nuget jobs is failing since we updated our release verification to use .NET 7.0

### What changes are included in this PR?

Update docker image used to use .NET 7.0

### Are these changes tested?

Will trigger jobs via archery

### Are there any user-facing changes?

No
* Closes: #34661

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>