uhttpd: Decrease the default validity time of certificate

uhttpd: Decrease the default validity time of certificate

The recommended maximum validity period is currently 397 days
and some browsers throw warning with longer periods.

Reference to
https://cabforum.org/working-groups/server/baseline-requirements/
 6.3.2 Certificate operational periods and key pair usage periods
 Subscriber Certificates issued on or after 1 September 2020
 SHOULD NOT have a Validity Period greater than 397 days and
 MUST NOT have a Validity Period greater than 398 days.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Link: https://github.com/openwrt/openwrt/pull/15366
Signed-off-by: Robert Marko <robimarko@gmail.com>