Remove Helm v2 and Tiller usage from cloud bootstrap scripts and docs

Summary

With GitLab 14.0 we move to Helm 3 and drop support for Helm 2. Our GKE, EKS, and AKS cloud installation docs and scripts support Helm 2 and Helm 3 behind a feature flag. We need to remove the Helm 2 and Tiller code and mentions from these files and docs.

scripts:

• https://gitlab.com/gitlab-org/charts/gitlab/blob/master/scripts/gke_bootstrap_script.sh
• https://gitlab.com/gitlab-org/charts/gitlab/blob/master/scripts/eks_bootstrap_script.sh
• https://gitlab.com/gitlab-org/charts/gitlab/blob/master/scripts/aks_bootstrap_script.sh

docs:

• https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/doc/installation/cloud/gke.md
• https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/doc/installation/cloud/eks.md
• https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/doc/installation/cloud/aks.md

As mentioned in !2002 (comment 584851237), we should also remove references in .markdownlint.json and spelling-exceptions.txt.

#2726 (closed) covers removing Helm 2 from the rest of this projects documentation.