[Docs][Release][Website] Figure out why the version banner code changed in 19.0.0

[amoeba]

After we released 19.0.0 and updated the website, I noticed the version banner wasn't set up right. Upon investigating, it looks like the post-10-docs.sh script failed to update the banner code on the stable docs and I didn't notice before merging. I captured more detail in apache/arrow-site#576.

When I grabbed the published version of the 19.0.0 docs, there was a newline that's not usually present in the version banner script:

    <script>
        DOCUMENTATION_OPTIONS.theme_version = '0.16.1';
        DOCUMENTATION_OPTIONS.theme_switcher_json_url = '/docs/_static/versions.json';
        DOCUMENTATION_OPTIONS.theme_switcher_version_match = '';
        DOCUMENTATION_OPTIONS.show_version_warning_banner =
            true; // <---- this is usually on the line above
        </script>

This caused the script's sed command to fail to replace the true with a false. If this happens again, the post-10-docs.sh script will fail so we should figure out what changed before the next release.