feat: update Helm to 3.7.1 and kubectl to 1.20

Helm and Kubectl

• Helm updated from v3.6.3 to v3.7.1 (see v3.7.0 release notes and v3.7.1 release notes)
• Kubectl updated to 1.20 because as-of %14.5, 1.19 will be GitLab's lowest supported version (and Kubernetes has N-1 API version compatibility)

Changes to CI

• Remove unsupported 1.16 and 1.17
• Add already supported 1.20
• Add 1.21 and 1.22 in anticipation of near-future support
  • Note: Kubectl had to be updated to from 1.17 to 1.20 in order to support ingress resources for 1.22, because 1.22 only has networking.k8s.io/v1, and this was first added in 1.19