chore(queue): remove system queue migrations (#4722)

## Description

Remove the remaining OSS surface for the retired `queue-migrate` system
queue path:

- remove `KindQueueMigrate` and its dev-server/deletion classifications
- remove the legacy `MigratePayload`, bulk `Migrate` API, and
implementation
- narrow the queue's migration interface to the still-active
migration-lock operation
- replace migration-kind test fixtures with an active system queue kind

Queue migration item movement now lives in the cloud migration
implementation. Migration locking remains in OSS because queue
processing still uses it to prevent work from starting during an active
migration.

This follows the cloud cleanup in inngest/monorepo#8172. That PR now
uses a private migration payload and no longer depends on the removed
OSS type.

## Release note

None

## Migration note

None

## Validation

- `go test -count=1 ./pkg/execution/queue
./pkg/execution/state/redis_state ./pkg/execution/executor
./pkg/execution/deletion ./tests/execution/queue`
- `git diff --check`