feat: handle kube loadBalancersSubnetId on cluster create and update

[kratisto]

Description

Add the possibility to define loadbalancers subnetid on the cluster creation or update.

I fixed a bug in the construction of the URL for the subnet deletion. Instead of using the private network id , the subnet id was used. This caused 404 errors during the acceptance tests TestAccCloudProjectKubeVRack

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Documentation update

How Has This Been Tested?

• [x] Test A: make testacc TESTARGS="-run TestAccCloudProjectKubeVRack"
• [x] Test B: make testacc TESTARGS="-run TestAccCloudProjectKubeNodesSubnetId"
• [x] Test C: make testacc TESTARGS="-run TestAccCloudProjectNetworkPrivateSubnet"

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings or issues
• [x] I have added acceptance tests that prove my fix is effective or that my feature works
• [x] New and existing acceptance tests pass locally with my changes
• [x] I ran succesfully go mod vendor if I added or modify go.mod file