[CI] Upgrade EKS cluster to 1.19

Summary

As part of adding support for Kubernetes 1.22 (gitlab-org&6883 (closed)), our NGINX upgrade (#2852 (closed)) requires at least Kubernetes 1.19.

As such, we'll need to update the EKS cluster, which is currently at version 1.18.20.

Follow-up to #2996 (closed).

Acceptance criteria

• EKS cluster is upgraded to version 1.19
• CI jobs running on EKS 1.19 pass