### 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
) {
        if (dryRun == false) {
            IndexLifecycleMetadata currentMetadata = currentState.metadata().custom(IndexLifecycleMetadata.TYPE);
            if (currentMetadata != null && currentILMMode(currentState) != STOPPED) {
                throw new IllegalStateException(
                    "stop ILM before migrating to data tiers, current state is [" + currentILMMode(currentState) + "]"
                );
            }
        }
        Metadata.Builder mb = Metadata.builder(currentState.metadata());
        
        Settings.Builder persistentSettingsBuilder = Settings.builder().put(mb.persistentSettings());
        persistentSettingsBuilder.remove(ENFORCE_DEFAULT_TIER_PREFERENCE);
        mb.persistentSettings(persistentSettingsBuilder.build());
        
        Settings.Builder transientSettingsBuilder = Settings.builder().put(mb.transientSettings());
        transientSettingsBuilder.remove(ENFORCE_DEFAULT_TIER_PREFERENCE);
        mb.transientSettings(transientSettingsBuilder.build());
        String removedIndexTemplateName = null;
        if (Strings.hasText(indexTemplateToDelete)) {
            if (currentState.metadata().getTemplates().containsKey(indexTemplateToDelete)) {
                mb.removeTemplate(indexTemplateToDelete);
                logger.debug("removing legacy template [{}]", indexTemplateToDelete);
                removedIndexTemplateName = indexTemplateToDelete;
            } else {
                logger.debug("legacy template [{}] does not exist", indexTemplateToDelete);
            }
        }
        String attribute = nodeAttrName;
        if (Strings.isNullOrEmpty(nodeAttrName)) {