Use Helm 3.7 instead of 3.4

What does this MR do?

Uses Helm 3.7.2 instead of 3.4.1 to support Kuberenetes 1.19.x to 1.22.x.

Also upgraded Ruby from 2.6.5 to 2.7.6 and kubectl from 1.16.4 to 1.20.15.

Due to a combination of support of Kubernetes 1.22 in !2267 (merged) and a limitation of helm-kubeval, this needs to bring some tweaks on validations on Kuberenetes 1.19 and 1.20. A limitation of helm-kubeval might be resolved in #2772 (closed).

• helm --kube-version 1.22.2 option, introduced in Helm 3.6.0: https://github.com/helm/helm/pull/9040, is documented at https://helm.sh/docs/helm/helm_template/ :

    --kube-version string                        Kubernetes version used for Capabilities.KubeVersion

Note on failed jobs (allowed_failure)

qa_gke jobs and qa_eks jobs are not relevant to this change. (Internal link: https://gitlab.slack.com/archives/C3JJET4Q6/p16547427580131090)

Related issues

Part of gitlab-org/gitlab-build-images#107 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• [n/a] Documentation created/updated
• Tests added
• [n/a] Integration tests added to GitLab QA
• [n/a] Equivalent MR/issue for omnibus-gitlab opened