GH-47258: [Release] Set date: for apache/arrow-site's _release/${VERSION}.md (#47260)

### Rationale for this change

We want to avoid needless Jekyll built site changes. If we don't specify date explicitly, the current time is used. It always causes some changes.

See also: apache/arrow-site#689

### What changes are included in this PR?

Set `date: ` explicitly.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: #47258

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Raúl Cumplido <[email protected]>

Author: kou

dev/release/post-04-website.sh

@@ -95,6 +95,7 @@ cat <<ANNOUNCE > "${announce_file}"
 ---
 layout: default
 title: Apache Arrow ${version} Release
+date: "${release_date_iso8601}"
 permalink: /release/${version}.html
 ---
 <!--