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GH-40814: [C++] Thirdparty: bump zstd to 1.5.6 (#40837)



### Rationale for this change

Zstd releases 1.5.6 here: https://github.com/facebook/zstd/releases/tag/v1.5.6

### What changes are included in this PR?

Change default zstd to 1.5.6

### Are these changes tested?

Already has test

### Are there any user-facing changes?

no

* GitHub Issue: #40814

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>