Assign Gitaly Review App pods to non-preemptible nodes

What does this MR do?

On nodes on preemptible VMs, pods are terminated without notice, which might explain why Gitaly has been transiently unavailable.

A non-preemptible node pool is configured in https://gitlab.com/gitlab-org/quality/engineering-productivity-infrastructure/-/merge_requests/12

See #335354 (comment 623451559)

Does this MR meet the acceptance criteria?

Tasks

• Create a new node pool (as configured in https://gitlab.com/gitlab-org/quality/engineering-productivity-infrastructure/-/merge_requests/12)

Conformity

• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.