Previously, the `aio_monitoring_stable` job (which runs tests against
https://angular.io/) was using the tests from the master branch. As a
result, if the master branch included changes in those tests that were
not yet backported to the stable branch (and thus deployed to
https://angular.io/), the tests would fail.
This commit fixes this by using the tests from the stable branch to test
against https://angular.io/.
Fixes#30101
PR Close#30110
George Kalpakas