Replace the mr-approved label with pipeline::tier-2/3 label

Context

We used pipeline:mr-approved label to separate predictive pipelines from "full" pipelines.

As part of gitlab-org/quality/engineering-productivity&58 (closed), we introduced pipeline tiers, and pipelinetier-2 would be able to replace pipeline:mr-approved.

Goals

1. pipeline:mr-approved should not be used to include/exclude CI jobs.
2. A pipelinetier-2 label should include all the jobs we need for a tier-2 pipeline.
3. A pipelinetier-3 label should include all the jobs we need for a tier-3 pipeline.

Communication

• Mention it in Slack, EWIR, ...