Use `ingressClassName:` instead of `kubernetes.io/ingress.class:`

What does this MR do and why?

The use of ingressClassName: is supported since 0.31.0 and in conformity with the networking.kubernetes.io/v1 API.

We're using the v1.1.0 of the app: https://gitlab.com/gitlab-org/quality/engineering-productivity-infrastructure/-/blob/1a26bb75715a26626cc4375e19dd1e5a789da173/qa-resources/modules/triage-reactive/main.tf#L46

From https://kubernetes.github.io/ingress-nginx/user-guide/basic-usage/:

Nginx is configured to automatically discover all ingress with the kubernetes.io/ingress.class: "nginx" annotation or where ingressClassName: nginx is present.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review