## What this PR does
Renames the misleading `subnets` field to `networks` in VMInstance. The
old field
name implied the VM was creating subnets, when in reality it attaches
the VM to
existing VPC network attachments (NetworkAttachmentDefinitions). This
change:
- Adds a new `networks` field as the primary way to attach VMs to VPC
networks
- Keeps `subnets` as deprecated with backward-compatible fallback via
Helm `default`
- Fails with a clear error if both `networks` and `subnets` are set
simultaneously
- Adds an API-backed dropdown selector for `networks[].name` listing
available
NetworkAttachmentDefinitions in the namespace
- Hides the deprecated `subnets` field from the dashboard UI
### Release note
```release-note
[vm-instance] Rename `subnets` field to `networks` in VMInstance for clarity.
The old `subnets` field is deprecated but still supported for backward compatibility.
A dropdown selector for available networks has been added to the dashboard.
```
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added native networks field for VM attachments and a UI dropdown to
select network attachments; UI hides deprecated subnet fields for VM
instances.
* **Deprecations**
* subnets is deprecated; use networks moving forward (subnets retained
for backward compatibility).
* **Chores / Migration**
* Added a migration to copy existing subnets into networks and bumped
platform migration target version.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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