Mark zoekt prometheus metric as deprecated

What does this MR do and why?

This MR marks the prometheus metric as deprecated. It is replaced by a gitlab-exporter metric (gitlab-org/ruby/gems/gitlab-exporter!216 (merged))

References

• Zoekt metrics from MetricsUpdateCronWorker are ... (#521933 - closed)
• Remove search_zoekt_task_processing_queue_size ... (#525102 - closed)
• Add zoekt probe (gitlab-org/ruby/gems/gitlab-exporter!216 - merged)

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How to set up and validate locally

docs only

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