Remove `herokuish` support from GitLab

Definition of done

Remove herokuish from auto-build-image:

• Add deprecation notice to https://docs.gitlab.com/ee/update/deprecations.html -> https://docs.gitlab.com/ee/update/deprecations.html#auto-devops-support-for-herokuish-is-deprecated
• Prepare a removal MR in https://gitlab.com/gitlab-org/cluster-integration/auto-build-image
• Remove herokuish documentation from monolith repository
• Create a follow-up issue for the next major milestone to merge the removal MR and update the version in the CI/CD templates

Leave:

• using Herokuish in Auto Test

References

• Slack discussion: https://cloud-native.slack.com/archives/C033DV8D9FB/p1689079292400619