chore: set default loadBalancerSKU to Standard

[nilo19]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

ClusterConfig defaults loadBalancerSku to basic: https://cloud-provider-azure.sigs.k8s.io/install/configs/#cluster-config.

On 30 September 2025, Basic SKU public IP addresses will be retired in Azure. You can continue to use your existing Basic SKU public IP addresses until then, however, you'll no longer be able to create new ones after 31 March 2025.

Basic SKU is being retired. We should not be defaulting to basic for LB and public IP SKUs.

Standard SKU public IP addresses offer significant improvements, including:

• Access to a variety of other Azure products, including Standard Load Balancer, Azure Firewall, and NAT Gateway.
• Security by default—closed to inbound flows unless allowed by a network security group.
• Zone-redundant and zonal front ends for inbound and outbound traffic.

Even if the retirement date is 3 years away, we should consider making this change now as the default value should reflect the recommended best practices in Azure.

Which issue(s) this PR fixes:

Fixes #2414

Special notes for your reviewer:

Does this PR introduce a user-facing change?

chore: set default loadBalancerSKU to Standard

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nilo19

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[lzhecheng]

/lgtm