GH-44558: [Release][Website] Remove needless "Apache Arrow ${VERSION}" section

[kou]

Rationale for this change

dev/release/post-04-website.sh generates wrong section levels:

...
## Changelog
## Apache Arrow 18.0.0 (2024-10-28 07:00:00+00:00)
...
### Bug Fixes
...
### New Features and Improvements
...

What changes are included in this PR?

Remove the ## Apache Arrow ${VERSION} section:

...
## Changelog
...
### Bug Fixes
...
### New Features and Improvements
...

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [Release][Website] Changelog section uses wrong section level #44558

[github-actions]

⚠️ GitHub issue #44558 has been automatically assigned in GitHub to PR creator.

[raulcd]

I've tested locally

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit fa7e0b4.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 1 possible false positive for unstable benchmarks that are known to sometimes produce them.