hwmon: (sht21) Update data sheet URLs

The previously used URLs are dead.

Sensirion provides permalinks for their product datasheets at
https://www.sensirion.com/en/about-us/links/.

Signed-off-by: Danilo Bargen <mail@dbrgn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>