[8.x](backport #4300) SLO roles privileges changed

[mergify]

Description

As described at https://github.com/elastic/observability-docs/issues/4229 in 8.16 the SLO permissions have changed, and also the builtin editor role is an option to grant privileges for SLOs.

This PR updates the slo-privileges.asciidoc and its associated images.
It also adds a minor change in serverless doc, as the required role to create SLOs is Editor and not Admin.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes https://github.com/elastic/observability-docs/issues/4229

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

---

This is an automatic backport of pull request #4300 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of bb910be has failed:

On branch mergify/bp/8.x/pr-4300
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit bb910bea.
  (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:
	deleted:    docs/en/observability/images/slo-es-priv-all.png
	new file:   docs/en/observability/images/slo-es-priv-editor.png
	deleted:    docs/en/observability/images/slo-es-priv-read.png
	new file:   docs/en/observability/images/slo-es-priv-viewer.png
	modified:   docs/en/observability/slo-privileges.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/slos/create-an-slo.mdx

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

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