Prepare release v0.27.0 - #678

#4254)

## What this PR does

Cluster-wide migration settings could only be applied by patching the
KubeVirt CR: the kubevirt chart did not render `migrations`, and the
platform bundle did not forward it. Add the optional
`kubevirt.migrations` platform value and pass it through the generated
Package to `spec.configuration.migrations`.

For example, set the following under `spec.components.platform.values`
on the `cozystack.cozystack-platform` Package:

```yaml
kubevirt:
  migrations:
    bandwidthPerMigration: 625M
    parallelMigrationsPerCluster: 2
    parallelOutboundMigrationsPerNode: 1
```

`bandwidthPerMigration` is a quantity in bytes per second. Empty or null
settings leave the field unset; existing KubeVirt defaults are
unchanged. The map is forwarded intact, including explicit `false` and
`0` values. `make update` restores the new parameterization
independently after a hand-merge.

Validation:

- All 207 Helm tests in `packages/system/kubevirt` and
`packages/core/platform` passed (42 suites).
- Before the template changes, the new limit and false/zero cases failed
at both forwarding stages (four expected regression failures).
- The GNU sed / make update-idempotency suite passed, including
restoration of a missing migrations block, repeated runs, and rejection
of a missing insertion anchor.
- Standalone KubeVirt rendering and `helm lint` passed. Neither modified
chart defines a `make generate` target. Full platform rendering requires
its live source repository lookup; the platform template behavior was
checked through its existing Helm unit-test fixtures.

This is separate from #3989, which exposes `vmStateStorageClass` in the
same chart and bundle files.


### Screenshots

Not applicable; no UI changes.

### Downstream repositories

- [ ] No downstream repository is affected by this change
- [x] [cozystack/website](https://github.com/cozystack/website) -
follow-up: [#697](cozystack/website#697)
- [ ]
[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
- follow-up:
- [ ]
[cozystack/ansible-cozystack](https://github.com/cozystack/ansible-cozystack)
- follow-up:
- [ ] [cozystack/ccp](https://github.com/cozystack/ccp) - follow-up:
- [ ] [cozystack/talm](https://github.com/cozystack/talm) - follow-up:
- [ ] [cozystack/cozyhr](https://github.com/cozystack/cozyhr) -
follow-up:
- [ ] [cozystack/cozy-proxy](https://github.com/cozystack/cozy-proxy) -
follow-up:
- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
- follow-up:
- [ ]
[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
- follow-up:
- [ ] [cozystack/examples](https://github.com/cozystack/examples) -
follow-up:
- [ ] [cozystack/community](https://github.com/cozystack/community) -
follow-up:

The website follow-up
[#697](cozystack/website#697) documents this new
platform value in the unreleased `next` reference and should be merged
after this implementation. The merged website PR
[#678](cozystack/website#678) documents
`kubevirt.disabledFeatureGates` only. The other listed repositories do
not need a coordinated change: this adds an optional platform value
without changing app schemas, required installer values, CRDs, package
layout, source kinds, shared tooling contracts, or node prerequisites.

### Release note

```release-note
feat(kubevirt): Configure cluster-wide KubeVirt migration settings through the platform Package value kubevirt.migrations. Empty settings preserve existing defaults.
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
  * Added optional KubeVirt migration configuration.
* Supports migration bandwidth, concurrency limits, auto-convergence,
and completion timeout settings.
  * Migration bandwidth can be specified in bytes per second.
  * Empty or unset migration settings preserve KubeVirt defaults.
* **Bug Fixes**
* Migration values now preserve valid `false` and `0` settings when
provided.
* **Tests**
* Added coverage for default, empty, null, configured, and
repeated-update scenarios.
* Added validation to prevent incomplete or duplicated migration
configuration.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->