[Development] PSA: Source code license checking will now happen with code from qtqa/dev not qtqa/master

[Alexandru Croitor]

Hi,

Once https://codereview.qt-project.org/c/qt/qt5/+/578757 merges, the CI will use the license checker code 
from the qtqa/dev branch rather than qtqa/master branch, for running the license checker in 
the dev and 6.8 branches of qt5.git and its submodules.

It will also use license checking coin instructions from qtbase, rather than instructions that were previously 
hardcoded in the Coin sources. This will allow us to do modifications without waiting for Coin updates.

Once we see that this works smoothly, the plan is then to start branching qtqa like we do for the rest of
our repos, so 6.9, 6.10, etc, instead of using one single master (or dev) branch for alllll qt5.git branches.

That should ease branch-specific changes, and also be closer to what qt developers expects.

The master branch continues to use the license checker from qtqa/master for the 5.15 branches
 and is run using the Coin-builtin instructions.

Thanks for reading, have a nice day.