Add a .gitreview to all repos

Adding a `.gitreview` file to all repos would help simplify the initial setup a user would need, and some IDEs integrate with it, e.g. to pick up the default branch to push to. Sample file:

[gerrit]
host=codereview.qt-project.org
project=qt/qtbase
defaultbranch=dev

This would also integrate with `git review` cli.

Reference: https://docs.opendev.org/opendev/git-review/latest/installation.html#gitreview-file-format