[8.16](backport #4594) Use official serverless product names

[mergify]

Description

Use official serverless product naming as described in #4455.

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 #4455

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 #4594 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f549fed has failed:

On branch mergify/bp/8.16/pr-4594
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit f549fed3.
  (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/en/observability/landing-page/page.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/apm-agents/apm-agents-opentelemetry-limitations.asciidoc
	deleted by us:   docs/en/serverless/apm-agents/apm-agents-opentelemetry.asciidoc
	deleted by us:   docs/en/serverless/apm/apm-ui-trace-sample-timeline.asciidoc
	deleted by us:   docs/en/serverless/cases/cases.asciidoc
	deleted by us:   docs/en/serverless/cases/manage-cases-settings.asciidoc
	deleted by us:   docs/en/serverless/index.asciidoc
	deleted by us:   docs/en/serverless/logging/log-monitoring.asciidoc
	deleted by us:   docs/en/serverless/observability-get-started.asciidoc
	deleted by us:   docs/en/serverless/projects/billing.asciidoc
	deleted by us:   docs/en/serverless/reference/elastic-entity-model.asciidoc
	deleted by us:   docs/en/serverless/synthetics/synthetics-get-started-project.asciidoc
	deleted by us:   docs/en/serverless/synthetics/synthetics-settings.asciidoc
	deleted by us:   docs/en/serverless/transclusion/apm/guide/diagrams/apm-otel-architecture.asciidoc
	deleted by us:   docs/en/serverless/transclusion/synthetics/global-managed-paid-for.asciidoc
	deleted by us:   docs/en/serverless/what-is-observability-serverless.asciidoc

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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