ci: fix multus installation

[zhangzujian]

• Make sure you have followed Kube-OVN Code Style.

What type of this PR

• CI

Which issue(s) this PR fixes:

Install the stable version in CI.

k8snetworkplumbingwg/multus-cni#1126

WHAT

🤖 Generated by Copilot at 81499b3

This pull request enhances the Makefile, the e2e test file, and the workflow file for the iptables-vpc-nat-gw feature. It makes the code more configurable, readable, and concise, and removes unnecessary steps and dependencies.

🤖 Generated by Copilot at 81499b3

We're sailing on the Kubernetes sea
We're simplifying our workflow with ENABLE_NAT_GW
We're updating our Makefile and our e2e test
We're heaving on the line, one, two, three, and rest

HOW

🤖 Generated by Copilot at 81499b3

• Simplify workflow steps by installing Kube-OVN and vpc-nat-gw in one command with ENABLE_NAT_GW flag (link)
• Introduce MULTUS_VERSION variable to specify and use consistent version of Multus CNI in workflow and image tag (link)
• Remove redundant kind-untaint-control-plane dependencies from kind-install-vpc-nat-gw and kind-install-lb-svc targets in Makefile (link, link)
• Add resource names to ginkgo.By messages in setupVpcNatGwTestEnvironment function in test/e2e/iptables-vpc-nat-gw/e2e_test.go (link)
• Replace hardcoded .kube-system suffix with framework.KubeOvnNamespace variable in ovn_eip package in test/e2e/iptables-vpc-nat-gw/e2e_test.go (link, link, link)
• Add ginkgo.By message to indicate overlay subnet retrieval in ovn_eip package in test/e2e/iptables-vpc-nat-gw/e2e_test.go (link)