ci(docs-infra): use the tests from the stable branch in `aio_monitoring_stable` CircleCI job (#30110)

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