### Elasticsearch Version

7.17, 8.x

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

Darwin

### Problem Description

The migrate to data tiers API can sometimes configure an illegal value for the `_tier_preference` setting for partially mounted indices.
This can happen if two conditions are met: 
1. if the partially mounted index doesn't have a `_tier_preference` configured
2. if the migrate to data tiers API doesn't perform any migration for this index (e.g. say this partially mounted index doesn't use the `data` custom attribute name for any allocation filtering, but either uses some other node attribute name or none at all)

In this case this piece of code kicks in to configure a default tier preference https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/cluster/metadata/MetadataMigrateToDataTiersRoutingService.java#L534

This is wrong, and we should configure the `MountSearchableSnapshotRequest.Storage.defaultDataTiersPreference` as the tier preference.

Relates to https://github.com/elastic/elasticsearch/issues/84758

### Steps to Reproduce

1. Mount an index - `partial-test`
2. Remove the `_tier_preference` using
```
PUT partial-test/_settings
{
  "index.routing.allocation.include._tier_preference": null
}
```
3. Call the migrate to data tiers routing API 
```
POST /_ilm/migrate_to_data_tiers
```

### Logs (if relevant)

_No response_
x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/cluster/metadata/MetadataMigrateToDataTiersRoutingService.java
final List<IndexMetadata> managedIndices = currentState.metadata()
            .indices()
            .values()
            .stream()
            .filter(meta -> policyName.equals(meta.getLifecyclePolicyName()))
            .toList();
        for (IndexMetadata indexMetadata : managedIndices) {
            LifecycleExecutionState currentExState = indexMetadata.getLifecycleExecutionState();
            if (currentExState != null) {
                Step.StepKey currentStepKey = Step.getCurrentStepKey(currentExState);
                if (currentStepKey != null && phasesWithoutAllocateAction.contains(currentStepKey.phase())) {
                    
                    if (currentStepKey.action().equals(AllocateAction.NAME)) {
                        
                        
                        LifecycleExecutionState newLifecycleState = moveStateToNextActionAndUpdateCachedPhase(
                            indexMetadata,
                            currentExState,
                            System::currentTimeMillis,
                            oldPolicy,
                            newPolicyMetadata,
                            client,
                            licenseState
                        );
                        if (currentExState.equals(newLifecycleState) == false) {
                            mb.put(IndexMetadata.builder(indexMetadata).putCustom(ILM_CUSTOM_METADATA_KEY, newLifecycleState.asMap()));
                        }
                    } else {