Remove SKE DNS gatewayApi API workaround after provider support

[lweberru]

Context

The SKE API supports extensions.dns.gatewayApi, but the Terraform provider schema currently does not expose a corresponding argument in stackit_ske_cluster.

Because of this gap, this repository currently uses a temporary API workaround:

• Terraform resource: terraform_data.dns_gateway_api_workaround
• Script: src/modules/platform-kubernetes/scripts/ske-enable-dns-gateway-api.sh
• Docs: docs/ske-dns-gateway-api-workaround.md

Exit criteria

• Terraform provider supports setting extensions.dns.gatewayApi natively.
• Module is migrated to the native provider argument.
• terraform_data.dns_gateway_api_workaround is removed.
• Script ske-enable-dns-gateway-api.sh is removed.
• Workaround flags are removed from DNS input.
• Documentation updated to native path only.
• This issue is closed.

References

• SKE API spec: services/ske/v2/ske.json (extensions.dns.gatewayApi)
• Provider resource docs: stackit_ske_cluster nested schema for extensions.dns