Use the UTF8 CSV file

[polm]

JP Post started distributing a UTF8 version of the main CSV file in June 2023.

https://www.post.japanpost.jp/zipcode/dl/utf-zip.html

Besides using UTF8, this replaces the weird line continuations with long lines, which is a welcome update. Unfortunately, it doesn't resolve other issues like comments in place names, and columns don't have headings. Also, no equivalent file is provided for the office postal codes.

It would be good to update posuto to use this new file after verifying that there's no difference from our conversion and the official one.