Remove SKE DNS gatewayApi API workaround after provider support

[lweberru]

Context

The SKE API supports extensions.dns.gatewayApi, but the Terraform provider currently does not expose this setting natively in stackit_ske_cluster.

Because of this gap, this repository currently uses a Terraform-native DNS record bridge (no script workaround):

• Endpoint discovery via Kubernetes data lookup (Envoy/Gateway service LB endpoint)
• DNS management via stackit_dns_record_set (A when IP is present, CNAME when hostname is present)
• Preconditions to avoid record creation before a usable endpoint is available

What is implemented today

• Gateway API with Envoy is used for routing.
• DNS is handled by Terraform resources, not by external scripts.
• The previous script-style workaround description is outdated and has been replaced.

Exit criteria

• Terraform provider supports setting extensions.dns.gatewayApi natively.
• Module is migrated from the temporary DNS record bridge to the native provider argument.
• Temporary bridge logic for endpoint discovery plus stackit_dns_record_set can be removed.
• Documentation is updated to the native path only.
• This issue is closed.

References

• SKE API capability: extensions.dns.gatewayApi
• Terraform resource in use for bridge: stackit_dns_record_set

[lweberru]

Reference for implementation context: #39

The current issue description now reflects the implemented Terraform DNS record bridge (stackit_dns_record_set) instead of the old script workaround narrative.