[8.5](backport #40275) [Metricbeat/kibana/status] Add support for v8format

[mergify]

Proposed commit message

Added the v8-format-compliant status.overall.level field in the metrics.
The v8 format was the default starting... ehem... v8.0.0 😅

While I was at it, I also added status.overall.summary for a human-friendlier explanation of the status.

Checklist

• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

The new field (and values) needs to be used in alerts/visualizations. Refer to elastic/kibana#76054 for more information about the breaking changes.

Related issues

• Relates Remove legacy /api/status endpoint response format kibana#94419

Use cases

Stack Monitoring.

cc @elastic/kibana-core @elastic/stack-monitoring

---

This is an automatic backport of pull request #40275 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 942f8c7 has failed:

On branch mergify/bp/8.5/pr-40275
Your branch is up to date with 'origin/8.5'.

You are currently cherry-picking commit 942f8c7857.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   metricbeat/docs/fields.asciidoc
	modified:   metricbeat/module/kibana/status/_meta/fields.yml
	new file:   metricbeat/module/kibana/status/_meta/testdata/8.16.0.json
	new file:   metricbeat/module/kibana/status/_meta/testdata/8.16.0.json-expected.json
	modified:   metricbeat/module/kibana/status/data.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   metricbeat/module/kibana/fields.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[botelastic]

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @afharo? 🙏