TPT-4368: Update integration tests due to deprecated "vpcs" for NB create

[mawilk90]

📝 Description

Changes related to the fact that "vpcs" attribute is deprecated (but still valid) in favor of "backend_vpcs" in POST /nodebalancers

✔️ How to Test

make test-int TEST_SUITE=nodebalancer TEST_ARGS="-k test_nb_with"

[Copilot]

Pull request overview

Updates NodeBalancer integration tests to align with the API change where the vpcs request attribute for POST /nodebalancers is deprecated in favor of backend_vpcs.

Changes:

• Replace vpcs= with backend_vpcs= in NodeBalancer create calls for backend-only VPC attachment.
• Replace vpcs= with backend_vpcs= in NodeBalancer create calls when backend and frontend VPCs are configured separately.

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[dawiddzhafarov]

What do you think of having a separate test case that still uses the deprecated "vpcs" argument and having a comment explaining that this test verifies it still works until its supported? Then, after it's not, we could remove it?

[mawilk90]

@dawiddzhafarov

What do you think of having a separate test case that still uses the deprecated "vpcs" argument and having a comment explaining that this test verifies it still works until its supported? Then, after it's not, we could remove it?

Yes, I will add something