Exclude CNI (secondary) subnets from EKS control plane VPC config

[mahmoudmohamed1990]

What type of PR is this?
/kind bug

What this PR does / why we need it:

EKS control plane ENIs could be placed in subnets tagged with
sigs.k8s.io/cluster-api-provider-aws/association=secondary, because
makeVpcConfig passes all subnets from the spec to EKS CreateCluster.
When the secondary CIDR is a non-routable range (common CGNAT pattern,
e.g. 100.64.0.0/16), this makes the private endpoint unreachable from
outside the VPC — including from the CAPA management cluster, which then
fails all workload-cluster reconciliation with connection timeouts.

This PR applies FilterNonCni() in makeVpcConfig, consistent with
nodegroup and EC2 instance placement (see #4800, which introduced the
helper for exactly this purpose but left the control plane path
unfiltered).

No effect on existing clusters — this only affects CreateCluster input;
reconcileVpcConfig does not reconcile SubnetIds.

Which issue(s) this PR fixes:
Fixes #6172

Special notes for your reviewer:
Added test cases to TestMakeVPCConfig covering CNI subnet exclusion.

Checklist:

• squashed commits
• includes documentation
• adds unit tests

​release-note EKS control plane ENIs are no longer placed in subnets tagged for CNI/pod networking (sigs.k8s.io/cluster-api-provider-aws/association=secondary) ​

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[kubernetes-prow]

Hi @mahmoudmohamed1990. Thanks for your PR.

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