use 'verified' review label for build bots

the git logs of qt and qt creator are littered by pointless reviewed-by footers from qt_ci_bot@qt-project.org, and qtc additionally from github-actions-qt-creator@cristianadam.eu.

this could be avoided by letting the bots do reviews under a separate label (for which footer generation is disabled) - gerrit's default configuration designates Verified for that. qbs already demonstrates this for qt-project; its travis-bot@weickelt.de doesn't show up in the logs.