qt: Prevent u32 overflow when calculating expiration date
* lang/qt/src/qgpgmesignkeyjob.cpp (sign_key): Change maxAllowedDate to 2106-02-05. Change log-level from warning to debug. * lang/qt/tests/t-various.cpp (TestVarious::testSignKeyWithExpiration): Remove check for warning. Adapt assertion.
Capping the expiration date at 2106-02-05 prevents a u32 overflow when
adding the number of days until the maximal date to the current time.
- GnuPG-bug-id: T5991