fix(ingress-nginx): Upgrade ingress-nginx to v1.12.1 and webhook certgen image to v1.5.2 (#12075)

This commit upgrades ingress-nginx to version v1.12.1, addressing multiple critical vulnerabilities including CVE-2025-1974, CVE-2025-1097, CVE-2025-1098, CVE-2025-24513, and CVE-2025-24514 as detailed in the ingress-nginx release notes: https://github.com/kubernetes/ingress-nginx/releases/tag/controller-v1.12.1

Important Notes:
- Fixing CVE-2025-1974 required disabling validation of the generated NGINX configuration during validation of Ingress resources. Invalid Ingress resources may stop the NGINX configuration from being updated.
- Recommended mitigations include enabling annotation validation and disabling snippet annotations.

Alongside this upgrade, the `ingress_nginx_kube_webhook_certgen_image_tag` has been updated to v1.5.2 for compatibility, based on: https://github.com/kubernetes/ingress-nginx/pull/13066

Changelog:
- Updated ingress-nginx version to v1.12.1 in Kubespray.
- Updated `ingress_nginx_kube_webhook_certgen_image_tag` in `roles/kubespray-defaults/defaults/main/download.yml` to v1.5.2.

Fixes: https://github.com/kubernetes-sigs/kubespray/issues/12073