[8.9](backport #3326) [synthetics] Add locations and private_locations lightweight options

[mergify]

This is an automatic backport of pull request #3326 done by Mergify.
Cherry-pick of a2f6d5e has failed:

On branch mergify/bp/8.9/pr-3326
Your branch is up to date with 'origin/8.9'.

You are currently cherry-picking commit a2f6d5e4.
  (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/synthetics-configuration.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/en/observability/synthetics-reference/lightweight-config/common.asciidoc

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

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