[Application Usage] Rollups: log.debug instead of log.warn

[afharo]

Summary

The users cannot do anything to solve the errors logged by Application Usage Rollup logic. So we shouldn't bother them with warning errors. This PR lowers the level down to debug so we can validate them when developing and testing.

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 1502047

Metrics [docs]

✅ unchanged

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@elasticmachine merge upstream