chore(slo): simplify error budget calculations

[kdelemme]

Resolves #158942

📝 Summary

Reason for change: https://elastic.slack.com/archives/C044PV8EJ4X/p1685717478850789

This PR updates the error budget calculation method for SLO setup with an occurrences budgeting method and calendar aligned time window, so we don't try to estimate the total number of events that will occur in the period.
This is important in order to support Composite SLO.

[apmmachine]

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[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 8fcd713

Metrics [docs]

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
securitySolution	414	418	+4
total			+6

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
securitySolution	498	502	+4
total			+6

History

• 💚 Build #132177 succeeded 6aba5c58b5d456ba2373d19b21c43f778ff30654

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@elasticmachine merge upstream

cc @kdelemme

[simianhacker]

LGTM