Disable jest coverage in stable branch pipelines

Problem

There is no reason to include Jest coverage in stable branch pipeline runs, as this coverage is already run in backport MRs, and only serves to increase pipeline runtime (~6m average per-job, per-pipeline)

Rel: gitlab-org/gitlab!164463 (merged)

Proposal

Ensure that jest coverage does not run in stable branch pipelines.

Disabling Jest coverage in stable branches would reduce 18.1, 18.0, and 17.11 and future stable branch pipeline times by 5-36% (saving 5.8-40 minutes per run) and eliminate flaky jobs that intermittently added ~59-minute delays, resulting in approximately 78 faster and more reliable pipelines per month. This change saves an estimated 90-624 hours monthly in CI/CD time while significantly improving developer productivity by reducing wait times and pipeline failures.