### 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.migratedIndices = Collections.unmodifiableList(migratedIndices);
            this.migratedPolicies = Collections.unmodifiableList(migratedPolicies);
            this.migratedTemplates = migratedTemplates;
        }
        @Override
        public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }
            MigratedEntities that = (MigratedEntities) o;
            return Objects.equals(removedIndexTemplateName, that.removedIndexTemplateName)
                && Objects.equals(migratedIndices, that.migratedIndices)
                && Objects.equals(migratedPolicies, that.migratedPolicies)
                && Objects.equals(migratedTemplates, that.migratedTemplates);
        }
        @Override
        public int hashCode() {
            return Objects.hash(removedIndexTemplateName, migratedIndices, migratedPolicies, migratedTemplates);
        }
    }
    /**
     * Represents the legacy, composable, and component templates that were migrated away from shard allocation settings based on custom
     * node attributes.
     */
    public static final class MigratedTemplates {
        public final List<String> migratedLegacyTemplates;
        public final List<String> migratedComposableTemplates;