### 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
* This returns `null` if an unknown attribute value is received.
     */
    @Nullable
    static String convertAttributeValueToTierPreference(String nodeAttributeValue) {
        String targetTier = "data_" + nodeAttributeValue;
        
        
        
        if (DataTier.validTierName(targetTier) == false || targetTier.equals(DataTier.DATA_CONTENT)) {
            return null;
        }
        return DataTier.getPreferredTiersConfiguration(targetTier);
    }
    /**
     * Represents the elasticsearch abstractions that were, in some way, migrated such that the system is managing indices lifecycles and
     * allocations using data tiers.
     */
    public static final class MigratedEntities {
        @Nullable
        public final String removedIndexTemplateName;
        public final List<String> migratedIndices;
        public final List<String> migratedPolicies;
        public final MigratedTemplates migratedTemplates;
        public MigratedEntities(
            @Nullable String removedIndexTemplateName,
            List<String> migratedIndices,
            List<String> migratedPolicies,
            MigratedTemplates migratedTemplates
        ) {
            this.removedIndexTemplateName = removedIndexTemplateName;