[ML] Explain log rate spikes: Disable UI/APIs.

[walterra]

Summary

Part of #136265.

This disables the UI and APIs for Explain log rate spikes in the ML plugin since it will not be part of 8.3. Once 8.3 has been branched off, we can reenable it in main. This also adds a check to the API integration tests to run the tests only when the hard coded feature flag is set to true.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

LGTM

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 9d31b1a
• Storybooks Preview
• Webpack Bundle Analyzer

Failed CI Steps

• FTR Configs #28
• FTR Configs #39

Test Failures

• [job] [logs] FTR Configs #39 / rules security and spaces enabled: basic ruleRegistryAlertsSearchStrategy logs should return alerts from log rules
• [job] [logs] FTR Configs #28 / security app - OIDC interactions URL capture "before all" hook for "can login preserving original URL"

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @walterra