### 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/test/java/org/elasticsearch/xpack/cluster/metadata/MetadataMigrateToDataTiersRoutingServiceTests.java
lifecycleName = randomAlphaOfLengthBetween(10, 15);
        indexName = randomAlphaOfLengthBetween(10, 15);
        client = mock(Client.class);
        logger.info("--> running [{}] with indexName [{}] and ILM policy [{}]", getTestName(), indexName, lifecycleName);
    }
    public void testMigrateIlmPolicyForIndexWithoutILMMetadata() {
        ShrinkAction shrinkAction = new ShrinkAction(2, null);
        AllocateAction warmAllocateAction = new AllocateAction(null, null, Map.of("data", "warm"), null, Map.of("rack", "rack1"));
        AllocateAction coldAllocateAction = new AllocateAction(0, null, null, null, Map.of("data", "cold"));
        SetPriorityAction warmSetPriority = new SetPriorityAction(100);
        LifecyclePolicyMetadata policyMetadata = getWarmColdPolicyMeta(
            warmSetPriority,
            shrinkAction,
            warmAllocateAction,
            coldAllocateAction
        );
        ClusterState state = ClusterState.builder(ClusterName.DEFAULT)
            .metadata(
                Metadata.builder()
                    .putCustom(
                        IndexLifecycleMetadata.TYPE,
                        new IndexLifecycleMetadata(
                            Collections.singletonMap(policyMetadata.getName(), policyMetadata),
                            OperationMode.STOPPED
                        )
                    )
                    .put(IndexMetadata.builder(indexName).settings(getBaseIndexSettings()))
                    .build()
            )
            .build();