Add nginx-ingress PDB maxUnavailable

What does this MR do?

This adds support for setting nginx-ingress PDB maxUnavailable instead of minAvailable.

In GitLab.com this is causing a small disruption during node rotation and upgrades.

❯ k get pdb -n gitlab gitlab-nginx-ingress-controller
NAME                              MIN AVAILABLE   MAX UNAVAILABLE   ALLOWED DISRUPTIONS   AGE
gitlab-nginx-ingress-controller   2               N/A               20                    359d

Related issues

gitlab-com/gl-infra/k8s-workloads/gitlab-com!1763 (merged)

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
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened