Eliminate duplicated or unneeded jobs for backport MRs and stable branches

Problem

Between the pre- and post-merge pipelines, there are duplicated or unneeded tests / jobs being run. This leads to slower pipelines overall for backport MRs.

Current suite strategy:
Gather current test suites and execution for re... (gitlab-org/quality/quality-engineering/team-tasks#3623 - closed)

Proposal

Identify and eliminate any duplicated or unneeded jobs to improve the pipeline execution time for backport MRs.

Tasks

• Remove jest vue3 check quarantined and jest-with-fixtures vue3 check quarantined from backport MRs (Runtime: ~17m)
• (If we didn't remove all jest jobs) Disable Jest coverage on stable branches (like we did in Disable jest coverage in MRs (gitlab-org/gitlab!164463 - merged) for merge requests) (could potentially save 5.8 minutes average per job)
• Remove graphql-verify job from stable branches (6m)
• [-] Identify unneeded jobs for CI changes. (context: for a simple CI change, and entire pipeline is run) Predictive test detection is ongoing on this track: gitlab-org/quality/analytics&12 (closed)
• Investigate and remove semgrep-appsec-custom-rules from backport MRs (MR)

Results

What was done	Effect
Fixed semgrep-appsec-custom-rules flaky issue.	Fixing this intermittent job eliminates both retry interruptions (which cost ~2-5 minutes plus engineer context-switching) and occasional 10-minute extended runs across 3 pipeline tiers. While the impact isn't as dramatic as full pipeline re-runs, preventing even 10-20 weekly failures still saves hours of compute time and eliminates disruptive engineer interruptions, improving both CI/CD efficiency and developer flow.
Removed two unnecessary jobs, jest vue3 check quarantined and jest-with-fixtures vue3 check quarantined from backport MRs	Removed two jobs totaling 17 minutes from backport MR pipelines, reducing Tier 1 and Tier 3 pipeline durations by ~16% and saving approximately 14 hours of compute time weekly.
Disabled Jest coverage in stable branches.	Disabling Jest coverage in stable branches reduces 18.1, 18.0, and 17.11 stable branch pipeline times by 5-36% (saving 5.8-40 minutes per run) and eliminated 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.