docs: Change lower datetime bound in doc/tests about limits

Although SQLite supports dates starting from year 0000-01-01 (1 BC
in ISO8601 parlance), we are porting to using GDateTime internally
which changes the lower boundary to 0001-01-01.

This date range change is unlikely to have real life consequences,
and OTOH using GDateTime allows us to drop our own ISO8601 parsing
and architecture dependent code, which has larger consequences with
eg. 32-bit time_t.