sync subnet to vpc while switching between custom VPC and default VPC

[qiutingjun]

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

What type of this PR

• Bug fixes

Which issue(s) this PR fixes:

Fixes #(issue-number)

WHAT

🤖 Generated by Copilot at 8432a48

Refactor and improve subnet and VPC controller logic. Use constants for annotation keys and handle subnet VPC changes and annotations in subnet.go. Follow Go naming and style conventions in subnet.go and vpc.go.

🤖 Generated by Copilot at 8432a48

Sing, O Muse, of the cunning code that changed the subnet's fate,
How util.VpcLastName and util.VpcLastPolicies were born,
To mark the shifting VPCs and their rules with annotations great,
As ./pkg/controller/subnet.go was refactored and reborn.

HOW

🤖 Generated by Copilot at 8432a48

• Declare multiple variables using short variable declaration blocks for brevity and consistency (link, link, link, link, link, link, link, link)
• Remove redundant declaration of usingIPs variable in pkg/controller/subnet.go (link)
• Add blank lines before assigning values from function calls or status fields for readability and consistency in pkg/controller/subnet.go (link, link, link)
• Rename variables and functions to use uppercase for acronyms for consistency with Go naming convention in pkg/controller/subnet.go (link, link, link, link, link, link, link, link, link, link, link, link, link, link)
• Add logic to set and check util.VpcLastName annotation on subnet objects to record and validate the previous VPC name of the subnet in pkg/controller/subnet.go (link, link, link)
• Remove extra blank line before logging the enqueue update subnet message in pkg/controller/subnet.go (link)
• Replace hard-coded annotation key "ovn.kubernetes.io/last_policies" with constant util.VpcLastPolicies for readability and maintainability in pkg/controller/vpc.go (link, link)
• Add constants util.VpcLastName and util.VpcLastPolicies to the package util in pkg/util/const.go (link)

[oilbeater]

The e2e fails

[qiutingjun]

The e2e fails

it's ok now.
It's strange that I ran an e2e test locally before and did not fail.