[8.16](backport #15953) Remove tf module for running soaktest and update readme

[mergify]

Motivation/summary

In #14411 we have removed the long-running soak testing setup. The PR also removes the now unused soak testing terraform script and repurposes the directory in the form of a guide for creating soak test workers.

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Documentation has been updated

How to test these changes

NA

Related issues

#14411

---

This is an automatic backport of pull request #15953 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of cf28a0d has failed:

On branch mergify/bp/8.16/pr-15953
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit cf28a0dc.
  (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:   testing/apmsoak/README.md
	deleted:    testing/apmsoak/main.tf
	deleted:    testing/apmsoak/terraform.tfvars.example
	deleted:    testing/apmsoak/variables.tf

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: testing/apmsoak/Makefile

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