Replace pipeline:mr-approved with tier labels in CI rules

What does this MR do and why?

Replace pipeline:mr-approved with tier labels in CI rules:

• Replace .if-merge-request-approved to check for pipeline::tier-[23] labels
• Replace .if-merge-request-not-approved with .if-merge-request-tier-1
• Rename rules to clarify intent: "approved" becomes "should-run-full-pipeline"

This removes dependency on the pipeline:mr-approved label while maintaining the same pipeline behavior using the tier system.

References

Contributes to Replace the mr-approved label with pipeline::ti... (#462820)

Proof of work

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