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GH-36513: [Dev][C#] Add Dependabot configuration for NuGet (#36514)

### Rationale for this change

We don't want to adjust PR titles from Dependabot manually.

### What changes are included in this PR?

Set the commit message prefix for `/csharp/` from Dependabot.
 
### Are these changes tested?

No. I hope that this works well...

### Are there any user-facing changes?

No.
* Closes: #36513

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>