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GH-45137: [CI][C++] Add a GCC 15 job (#45138)

### Rationale for this change

GCC 15 isn't released yet but we'll be able to add support for the latest GCC in a timely manner with this.

### What changes are included in this PR?

Add a GCC 15 job based on Debian GNU/Linux experimental.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #45137

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>