Reference NGINX Ingress Controller v1.2.1

What does this MR do?

Reference NGINX Ingress Controller v1.2.1

References version 1.2.1 of the NGINX Ingress Controller, mirrored in
https://gitlab.com/gitlab-org/cloud-native/mirror/images/-/merge_requests/6.

Digest was collected via:

```
$ skopeo inspect docker://k8s.gcr.io/ingress-nginx/controller:v1.2.1 | jq -r '.Digest'
sha256:5516d103a9c2ecc4f026efbd4b40662ce22dc1f824fb129ed121460aaa5c47f8
```

Related issues

Closes #3403 (closed)

Testing

Confirming that the image pulls from our mirror in review environment:

[cluster: gke-cloud-native]
$ kubectl get pods -l release=rvw-3403-reference-nginx-1-2-1,app=nginx-ingress -oyaml | yq '.items[0].spec.containers[0].image'
registry.gitlab.com/gitlab-org/cloud-native/mirror/images/ingress-nginx/controller:v1.2.1@sha256:5516d103a9c2ecc4f026efbd4b40662ce22dc1f824fb129ed121460aaa5c47f8

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
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

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
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

Closes #3403 (closed)