facilitate community maintenance of qt 5.15

following tuukka's announcement [1], i request that in all repos that are locked down, a branch named 5.15-free is forked from the last public 5.15 commit. the pattern follows what was established in the qt/qt repository; "free" seems like an appropriate suffix.

i presume tqtc will not be willing to run the CI gate on this branch, so hopefully KDAB or another entity will attach an external build bot.

[1] https://lists.qt-project.org/pipermail/development/2021-January/040798.html