Bump gitlab-labkit to 5.0.0

What does this MR do and why?

Bump gitlab-labkit to 5.0.0

labkit 5.0.0's only behavior change is in Labkit::RateLimit: it removes the deprecated gitlab_labkit_rate_limiter_calls_total and gitlab_labkit_rate_limiter_errors_total counters, already superseded in 4.x by per-check and per-rule-evaluation counters, and adds gitlab_labkit_rate_limiter_peeks_total{rate_limiter, error}, emitted once per #peek call including calls that fail open. Nothing in this repo reads the removed counters; two comments referencing the old name were updated to match.

Relates to gitlab-com/gl-infra/production-engineering#29519 (closed)

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