Fix wxDateTime::GetWeekOfYear() for the days in the last week of the year.

Fix wxDateTime::GetWeekOfYear() for the days in the last week of the year.

The code took into account the possibility that the days in the beginning of
the year might belong to the last week of the previous year but not that the
days at the end of the year could belong to the first week of the next year.

Closes #14973.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73402 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775