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GH-37984: [Release] Use ISO 8601 format for YAML date value (#37985)

### Rationale for this change

If we use ISO 8601 format for date value, we can use the value as a `Date` object in Jekyll. It's easy to reuse.

Use case: https://github.com/apache/arrow-site/pull/408

### What changes are included in this PR?

Use ISO 8601 format.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.
* Closes: #37984

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