/kind bug

What steps did you take and what happened:
When creating a cluster with provided network, which has multiple subnets, and a provided subnet id the wrong subnet is used.

What did you expect to happen:
The subnet with the id provided is used instead of the default one.

Anything else you would like to add:
This is related to the issue #2537.
I would like to contribute with a patch and refering to the contributing guidelines i had to create an issue. This is done :)
I already made a patch and tested it in my environment.
As expressed in the environment paragraph it has been patched and tested on the v0.11.4 version.
Do i have to redo on main branch to provide the patch ?

[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster API Provider OpenStack version (Or git rev-parse HEAD if manually built): v0.11.4
• Cluster-API version: v1.9.1
• OpenStack version: 2024.2 (dalmatian)
• Minikube/KIND version:
• Kubernetes version (use kubectl version): 1.32.4
• OS (e.g. from /etc/os-release): alpine