[Reg 6.6.1->6.6.2] Performance regression in qhelpcollection.cpp

The change https://codereview.qt-project.org/c/qt/qttools/+/527983 introduced an issue where timestamps are stored in UTC and later compared (as a string) in the local timezone. Due to that the comparison always fails which leads to every documentation file be unregistered and re-registered. This leads to noticable hangs in Qt Creator during startup and when opening the help view.

https://codereview.qt-project.org/c/qt/qttools/+/545125 fixes this issue.