Replaced stage cleanup (as gitlab doesn't have it anymore, since around version 14 or 15) for .post

What does this MR do and why?

Replaced stage "cleanup" for ".post". I've realized the stage cleanup as removed from gitlab as we did a local gitlab update to 15.4.2 (previous version was 13.3). Since then the extended stage we had for terraform destroy stopped working with error:

Found errors in your .gitlab-ci.yml:
    infra-destroy job: chosen stage does not exist; available stages are .pre, validate, test, build, deploy, .post

And

Found errors in your .gitlab-ci.yml:
    infra-destroy: unknown keys in `extends` (.destroy)

Screenshots or screen recordings

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How to set up and validate locally

It can be tested including the template in your terraform gitlab-ci and extending it, eg:

infra-destroy:
  extends: .terraform:destroy

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