unicode: change the reference of database file

Commit 2b3d0478 ("unicode: Add utf8-data module") changed
the database file from 'utf8data.h' to 'utf8data.c' to build
separate module, but it seems forgot to update README.utf8data
, which may causes confusion. Update the README.utf8data and
the default 'UTF8_NAME' in 'mkutf8data.c'.

Signed-off-by: Gan Jie <ganjie182@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240912031932.1161-1-ganjie182@gmail.com


Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>