mgr/devicehealth: fix missing timezone from time delta calculation

mgr/devicehealth: fix missing timezone from time delta calculation

An error occurs when subtracting a datetime object that is offset-naive
(i.e. unaware of timezone) from a datetime object which is offset-aware.

datetime.utcnow() is missing timezone info, e.g.:
'2021-09-22 13:18:45.021712',
while life_expectancy_max is in the format of:
'2021-09-28 00:00:00.000000+00:00',
hence we need to add timezone info to the former when calculating
their time delta.

Please note that we calculate time delta using `datetime.utcnow()` in
`serve()` in this module, but there we refer to the delta in seconds,
which works fine.

Fixes: https://tracker.ceph.com/issues/52327
Signed-off-by: Yaarit Hatuka <yaarit@redhat.com>
(cherry picked from commit 05902d943bba4a64abbd943270b56cbdd1650e62)