Incorrect runbook link for alert `service_apdex_slo_out_of_bounds_lower_15m`

Link for runbook ("troubleshooting/service-ci-runners.md") on service_apdex_slo_out_of_bounds_lower_15m alert is incorrect.

Correct link is: https://ops.gitlab.net/gitlab-com/runbooks/-/blob/master/docs/ci-runners/service-ci-runners.md