Fix rules functional test by replacing uptime rule with metric threshold

[maryam-saeidi]

Fixes #163427

Summary

Fix rules functional test by replacing uptime rule with a metric threshold as uptime is now replaced with synthetics (Why understand and solve an issue if we can remove it altogether?! :D)

Flaky test runner [100]: https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/2859

[apmmachine]

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• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• /oblt-deploy-serverless : Deploy a serverless Kibana instance using the Observability test environments.
• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[kibana-ci]

💚 Build Succeeded

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• Commit: 2c81749

Metrics [docs]

✅ unchanged

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@elasticmachine merge upstream

cc @maryam-saeidi