Certmanager: Enable Gateway API by default

What does this MR do?

Certmanager: Enable Gateway API by default

In !4953 (merged) Gateway API was enabled by default in favor of Ingress.

To accommodate these changes, the bundled certmanager should also be configured for Gateway API.

Relates #6414 (closed)

References

• Our migration docs: https://docs.gitlab.com/charts/installation/migration/envoy_gateway_migration/#migrate-in-one-step
• Upstream docs: https://cert-manager.io/docs/usage/gateway/

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• Documentation created/updated.
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• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
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