Add maintainers as reviewers to gerrit changes automatically

As per QTQAINFRA-169, we now have the reviewers plugin installed.

The docs are here https://codereview.qt-project.org/plugins/reviewers/Documentation/config.md

Now somebody needs to actually add reviewers and filters to relevant projects.

Recently someone on IRC mentioned it would be great if reviewers are automatically added to changes, instead of them looking through git blame to find the right people.

I suggest we at least add the maintainers from https://wiki.qt.io/Maintainers as reviewers for each respective repository, limited to a specific subdirectory if needed.

We already have some reviewers for the qt5.git/coin folder for more than half a year, and that seems to work fine.

To make it more /maintainable/ (pun intended), we can create separate "Qt Project Maintainers - Qt Foo module" gerrit groups, assign maintainers to them, and then create filters using those groups, similar to the qt5.git CI review groups https://codereview.qt-project.org/c/qt/qt5/+/591326/5/reviewers.config

Then when a new maintainer is appointed or leaves, we just modify the respective maintainer group.