[8.15] Case custom fields GA (backport #5591)

[mergify]

Relates to elastic/kibana#188598

This PR updates the Observability case documentation related to custom fields moving from "tech preview" to "GA" state in 8.15.0. It also moves the existing custom fields content to the updated case settings page.

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This is an automatic backport of pull request #5591 done by Mergify.

[mergify]

Cherry-pick of 294e0f5 has failed:

On branch mergify/bp/8.15/pr-5591
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit 294e0f51.
  (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:   docs/cases/cases-manage.asciidoc
	deleted:    docs/cases/images/cases-custom-field-settings.png
	new file:   docs/serverless/images/cases-settings/security-cases-custom-fields.png

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   docs/cases/cases-ui-integrations.asciidoc
	deleted by us:   docs/serverless/investigate/cases-open-manage.mdx
	deleted by us:   docs/serverless/investigate/cases-settings.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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[nastasha-solomon]

@lcawl this anchor name is replicated in the topic anchor here -- or maybe it's the other way around? How did you want to handle this, and do we need to fix this in any other branches?

[mergify]

This pull request is now in conflicts. Could you fix it @mergify[bot]? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.15/pr-5591 upstream/mergify/bp/8.15/pr-5591
git merge upstream/8.15
git push upstream mergify/bp/8.15/pr-5591