Update Unicode download URL

Seems I forgot to update the unicode_download.sh script when I upgraded
from Unicode 14.0 to 15.0 in November 2023.

Author: bnoordhuis

unicode_download.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -e
 
-url="ftp://ftp.unicode.org/Public/14.0.0/ucd"
+url="ftp://ftp.unicode.org/Public/15.0.0/ucd"
 emoji_url="${url}/emoji/emoji-data.txt"
 
 files="CaseFolding.txt DerivedNormalizationProps.txt PropList.txt \