[Release][C#] Use .NET 3.1 for verification

[asfimport]

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #9254

_{Note: This issue was originally created as ARROW-11309. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 9254
#9254