[Development] qtbase 5.9 is locked down for the time being

[Liang Qi]

Hi, all,

Since Oct. 20, only one change landed in qtbase 5.9, https://codereview.qt-project.org/#/c/209093/

I will try to do manual stage in qtbase 5.9 to check whether it is because flakiness test or others.

—Liang