[8.15] fix(slo): Use correct calendar period (#199873)

[kibanamachine]

Backport

This will backport the following commits from main to 8.15:

• fix(slo): Use correct calendar period (#199873)

Questions ?

Please refer to the Backport tool documentation

[github-actions]

🤖 GitHub comments

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Just comment with: - `/oblt-deploy` : Deploy a Kibana instance using the Observability test environments. - `run` `docs-build` : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: d94e599

Failed CI Steps

• FTR Configs #46

Test Failures

• [job] [logs] FTR Configs #46 / saved objects tagging - functional tests dashboard integration creating allows to create a tag from the tag selector

Metrics [docs]

✅ unchanged

cc @kdelemme